modem: Cellular modem sample fails with RAK5010 board (quectel bg95 modem)

[olalonde]

Describe the bug

It seems there is an issue with the quectel bg95 modem driver at the AT+CREG? command. It gets stuck there and times out.

To Reproduce

Build and flash the cellular modem sample on a RAK5010-M-BG95 board.

I'm using a valid Claro (Dominican Republic) SIM card in an area with EGPRS signal.

Expected behavior

L4 should connect in time.

Impact

showstopper

Logs and console output

SEGGER J-Link V7.94i - Real time terminal output
SEGGER J-Link (unknown) V2.0, SN=802000342
Process: JLinkExe
*** Booting nRF Connect SDK v2.9.1-60d0d6c8d42d ***
*** Using Zephyr OS v3.7.99-ca954a6216c9 ***
Powering on modem
Bring up network interface
Waiting for L4 connected
[00:00:11.916,015] <dbg> modem_chat: modem_chat_script_start: running script: quectel_bg95_init_chat_script
[00:00:11.916,046] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_init_chat_script: step: 0
[00:00:11.916,076] <dbg> modem_chat: modem_chat_script_next: sending: ATE0
[00:00:21.916,137] <wrn> modem_chat: quectel_bg95_init_chat_script: timed out
[00:00:25.338,958] <dbg> modem_chat: modem_chat_on_unknown_command_received: RDY
[00:00:27.911,743] <dbg> modem_chat: modem_chat_on_unknown_command_received: APP RDY
[00:00:31.877,288] <dbg> modem_chat: modem_chat_on_unknown_command_received: RDY
[00:00:33.416,473] <dbg> modem_chat: modem_chat_script_start: running script: quectel_bg95_init_chat_script
[00:00:33.416,534] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_init_chat_script: step: 0
[00:00:33.416,564] <dbg> modem_chat: modem_chat_script_next: sending: ATE0
[00:00:33.437,744] <dbg> modem_chat: modem_chat_on_unknown_command_received: ATE0
[00:00:33.437,805] <dbg> modem_chat: modem_chat_log_received_command: OK
[00:00:33.437,835] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_init_chat_script: step: 1
[00:00:33.437,866] <dbg> modem_chat: modem_chat_script_next: sending: AT+CFUN=4
[00:00:33.572,387] <dbg> modem_chat: modem_chat_log_received_command: OK
[00:00:33.572,418] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_init_chat_script: step: 2
[00:00:33.572,448] <dbg> modem_chat: modem_chat_script_next: sending: AT+CMEE=1
[00:00:33.594,146] <dbg> modem_chat: modem_chat_log_received_command: OK
[00:00:33.594,207] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_init_chat_script: step: 3
[00:00:33.594,238] <dbg> modem_chat: modem_chat_script_next: sending: AT+CREG=1
[00:00:33.615,936] <dbg> modem_chat: modem_chat_log_received_command: OK
[00:00:33.615,966] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_init_chat_script: step: 4
[00:00:33.615,997] <dbg> modem_chat: modem_chat_script_next: sending: AT+CGREG=1
[00:00:33.637,695] <dbg> modem_chat: modem_chat_log_received_command: OK
[00:00:33.637,725] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_init_chat_script: step: 5
[00:00:33.637,756] <dbg> modem_chat: modem_chat_script_next: sending: AT+CEREG=1
[00:00:33.659,454] <dbg> modem_chat: modem_chat_log_received_command: OK
[00:00:33.659,484] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_init_chat_script: step: 6
[00:00:33.659,515] <dbg> modem_chat: modem_chat_script_next: sending: AT+CREG?
[00:00:33.681,091] <dbg> modem_chat: modem_chat_log_received_command: +CREG:  1 0
[00:00:33.681,182] <dbg> modem_chat: modem_chat_log_received_command: OK
[00:00:33.681,213] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_init_chat_script: step: 7
[00:00:33.681,243] <dbg> modem_chat: modem_chat_script_next: sending: AT+CEREG?
[00:00:33.702,911] <dbg> modem_chat: modem_chat_log_received_command: +CEREG:  1 4
[00:00:33.702,972] <dbg> modem_chat: modem_chat_log_received_command: OK
[00:00:33.703,002] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_init_chat_script: step: 8
[00:00:33.703,033] <dbg> modem_chat: modem_chat_script_next: sending: AT+CGREG?
[00:00:33.724,731] <dbg> modem_chat: modem_chat_log_received_command: +CGREG:  1 0
[00:00:33.724,792] <dbg> modem_chat: modem_chat_log_received_command: OK
[00:00:33.724,822] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_init_chat_script: step: 9
[00:00:33.724,853] <dbg> modem_chat: modem_chat_script_next: sending: AT+CGSN
[00:00:33.746,612] <dbg> modem_chat: modem_chat_log_received_command:  864593056768822
[00:00:33.746,643] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_init_chat_script: step: 10
[00:00:33.746,704] <dbg> modem_chat: modem_chat_log_received_command: OK
[00:00:33.746,704] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_init_chat_script: step: 11
[00:00:33.746,765] <dbg> modem_chat: modem_chat_script_next: sending: AT+CGMM
[00:00:33.768,218] <dbg> modem_chat: modem_chat_log_received_command:  BG95-M3
[00:00:33.768,280] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_init_chat_script: step: 12
[00:00:33.768,341] <dbg> modem_chat: modem_chat_log_received_command: OK
[00:00:33.768,341] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_init_chat_script: step: 13
[00:00:33.768,371] <dbg> modem_chat: modem_chat_script_next: sending: AT+CGMI
[00:00:33.789,855] <dbg> modem_chat: modem_chat_log_received_command:  Quectel
[00:00:33.789,886] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_init_chat_script: step: 14
[00:00:33.789,947] <dbg> modem_chat: modem_chat_log_received_command: OK
[00:00:33.789,978] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_init_chat_script: step: 15
[00:00:33.790,008] <dbg> modem_chat: modem_chat_script_next: sending: AT+CGMR
[00:00:33.811,492] <dbg> modem_chat: modem_chat_log_received_command:  BG95M3LAR02A03
[00:00:33.811,523] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_init_chat_script: step: 16
[00:00:33.811,584] <dbg> modem_chat: modem_chat_log_received_command: OK
[00:00:33.811,614] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_init_chat_script: step: 17
[00:00:33.811,645] <dbg> modem_chat: modem_chat_script_next: sending: AT+CIMI
[00:00:33.833,374] <dbg> modem_chat: modem_chat_log_received_command:  370021201019608
[00:00:33.833,404] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_init_chat_script: step: 18
[00:00:33.833,465] <dbg> modem_chat: modem_chat_log_received_command: OK
[00:00:33.833,465] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_init_chat_script: step: 19
[00:00:33.833,496] <dbg> modem_chat: modem_chat_script_next: sending: AT+QCCID
[00:00:33.855,224] <dbg> modem_chat: modem_chat_log_received_command: +QCCID:  89010210524010196085
[00:00:33.855,255] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_init_chat_script: step: 20
[00:00:33.855,346] <dbg> modem_chat: modem_chat_log_received_command: OK
[00:00:33.855,377] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_init_chat_script: step: 21
[00:00:33.855,407] <dbg> modem_chat: modem_chat_script_next: sending: AT+CMUX=0,0,5,127
[00:00:33.877,685] <dbg> modem_chat: modem_chat_on_unknown_command_received: OK
[00:00:34.155,548] <dbg> modem_chat: modem_chat_script_stop: quectel_bg95_init_chat_script: complete
[00:00:34.277,191] <inf> modem_at_shell: pipe connected
[00:00:34.277,252] <inf> modem_at_shell: opening pipe
[00:00:34.299,133] <inf> modem_at_shell: pipe opened
[00:00:34.299,194] <inf> modem_at_shell: chat attached
[00:00:34.420,684] <dbg> modem_chat: modem_chat_script_start: running script: quectel_bg95_dial_chat_script
[00:00:34.420,715] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_dial_chat_script: step: 0
[00:00:34.420,745] <dbg> modem_chat: modem_chat_script_next: sending: AT+CGACT=0,1
[00:00:34.443,969] <dbg> modem_chat: modem_chat_log_received_command: OK
[00:00:34.444,000] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_dial_chat_script: step: 1
[00:00:34.444,030] <dbg> modem_chat: modem_chat_script_next: sending: AT+CGDCONT=1,"IP","internet.ideasclaro.com.do"
[00:00:34.471,679] <dbg> modem_chat: modem_chat_log_received_command: OK
[00:00:34.471,710] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_dial_chat_script: step: 2
[00:00:34.471,740] <dbg> modem_chat: modem_chat_script_next: sending: AT+CFUN=1
[00:00:34.495,239] <dbg> modem_chat: modem_chat_log_received_command: OK
[00:00:34.495,269] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_dial_chat_script: step: 3
[00:00:34.495,300] <dbg> modem_chat: modem_chat_script_next: sending: ATD*99***1#
[00:00:34.495,422] <dbg> modem_chat: modem_chat_script_stop: quectel_bg95_dial_chat_script: complete
[00:00:34.519,073] <dbg> modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x0d because 0x7e was expected.
[00:00:34.519,073] <dbg> modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x0a because 0x7e was expected.
[00:00:34.519,104] <dbg> modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x43 because 0x7e was expected.
[00:00:34.519,134] <dbg> modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x4f because 0x7e was expected.
[00:00:34.519,134] <dbg> modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x4e because 0x7e was expected.
[00:00:34.519,165] <dbg> modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x4e because 0x7e was expected.
[00:00:34.519,165] <dbg> modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x45 because 0x7e was expected.
[00:00:34.519,195] <dbg> modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x43 because 0x7e was expected.
[00:00:34.519,195] <dbg> modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x54 because 0x7e was expected.
[00:00:34.519,226] <dbg> modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x20 because 0x7e was expected.
[00:00:34.519,226] <dbg> modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x31 because 0x7e was expected.
[00:00:34.519,256] <dbg> modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x35 because 0x7e was expected.
[00:00:34.519,256] <dbg> modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x30 because 0x7e was expected.
[00:00:34.519,287] <dbg> modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x30 because 0x7e was expected.
[00:00:34.519,287] <dbg> modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x30 because 0x7e was expected.
[00:00:34.519,317] <dbg> modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x30 because 0x7e was expected.
[00:00:34.519,317] <dbg> modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x30 because 0x7e was expected.
[00:00:34.519,348] <dbg> modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x30 because 0x7e was expected.
[00:00:34.519,348] <dbg> modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x30 because 0x7e was expected.
[00:00:34.519,378] <dbg> modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x0d because 0x7e was expected.
[00:00:34.519,409] <dbg> modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x0a because 0x7e was expected.
[00:00:34.908,691] <dbg> modem_chat: modem_chat_on_unknown_command_received: APP RDY
[00:00:34.908,782] <dbg> modem_chat: modem_chat_on_unknown_command_received: APP RDY
[00:00:37.650,604] <dbg> modem_chat: modem_chat_log_received_command: +CREG:  2
[00:00:37.650,726] <dbg> modem_chat: modem_chat_log_received_command: +CGREG:  2
[00:00:37.678,405] <dbg> modem_chat: modem_chat_log_received_command: +CREG:  1
[00:00:37.702,667] <dbg> modem_ppp: modem_ppp_process_received_byte: Receiving PPP frame
[00:00:37.702,880] <dbg> modem_ppp: modem_ppp_process_received_byte: Received PPP frame (len 29)
[00:00:37.703,308] <dbg> modem_ppp: modem_ppp_process_received_byte: Receiving PPP frame
[00:00:37.703,399] <dbg> modem_ppp: modem_ppp_process_received_byte: Received PPP frame (len 12)
[00:00:37.729,919] <dbg> modem_ppp: modem_ppp_process_received_byte: Receiving PPP frame
[00:00:37.730,010] <dbg> modem_ppp: modem_ppp_process_received_byte: Received PPP frame (len 13)
[00:00:37.753,875] <dbg> modem_ppp: modem_ppp_process_received_byte: Receiving PPP frame
[00:00:37.753,997] <dbg> modem_ppp: modem_ppp_process_received_byte: Received PPP frame (len 12)
[00:00:37.781,829] <dbg> modem_ppp: modem_ppp_process_received_byte: Receiving PPP frame
[00:00:37.781,951] <dbg> modem_ppp: modem_ppp_process_received_byte: Received PPP frame (len 12)
[00:00:37.782,012] <dbg> modem_ppp: modem_ppp_process_received_byte: Receiving PPP frame
[00:00:37.782,104] <dbg> modem_ppp: modem_ppp_process_received_byte: Received PPP frame (len 9)
[00:01:07.678,649] <dbg> modem_chat: modem_chat_script_start: running script: quectel_bg95_periodic_chat_script
[00:01:07.678,680] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_periodic_chat_script: step: 0
[00:01:07.678,710] <dbg> modem_chat: modem_chat_script_next: sending: AT+CREG?
[00:01:11.678,802] <wrn> modem_chat: quectel_bg95_periodic_chat_script: timed out
[00:01:41.678,985] <dbg> modem_chat: modem_chat_script_start: running script: quectel_bg95_periodic_chat_script
[00:01:41.679,046] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_periodic_chat_script: step: 0
[00:01:41.679,077] <dbg> modem_chat: modem_chat_script_next: sending: AT+CREG?
[00:01:45.679,138] <wrn> modem_chat: quectel_bg95_periodic_chat_script: timed out
L4 was not connected in time

Environment (please complete the following information):

• OS: Macos
• Toolchain: Zephyr SDK / nordic vscode extension
• Commit SHA or Version used: Zephyr OS v3.7.99-ca954a6216c9

Additional context

See discussion: #88678

[github-actions]

Hi @olalonde! We appreciate you submitting your first issue for our open-source project. 🌟

Even though I'm a bot, I can assure you that the whole community is genuinely grateful for your time and effort. 🤖💙

[olalonde]

After analyzing the logs with ChatGPT:

⚠️ Where It Starts Going Wrong
After the ATD99**1# command:

[00:00:12.979,980] sending: ATD*99***1#
[00:00:12.980,133] script: complete
[00:00:13.003,784] Dropping byte 0x0d because 0x7e was expected.
[...lots of dropped bytes...]

The log then shows many dropped bytes because the modem is sending text data (likely a CONNECT response or garbage characters), but the PPP handler is expecting the PPP framing byte 0x7e.

This strongly suggests:

The modem did not properly enter PPP mode, or

CONNECT message or similar was interpreted as data, confusing the PPP handler, or

The ATD99**1# did not result in an actual data session, perhaps because of no network registration or PDP context failure.

Earlier in the init and periodic scripts, you can see this:

+CREG:  1,0  → registered to home network (OK for CS)
+CGREG: 1,0  → registered for GPRS (OK for PS)
+CEREG: 1,4  → **not registered, and searching** (critical)
+CEREG: 1,4 means EPS (LTE/4G) registration is still in progress or failing.

This suggests the modem:

Hasn't fully attached to the LTE network

Likely cannot establish a data session when the PPP is triggered.

I know that there is not LTE-m or nb-iot in Dominican Republic so I expect the modem to connect to a 2G network. Does the driver only support connecting to a LTE network?

[0x447a]

@olalonde the cellular network supported are defined in an enum and are delcared as such:

enum cellular_access_technology {
CELLULAR_ACCESS_TECHNOLOGY_GSM = 0,
CELLULAR_ACCESS_TECHNOLOGY_GPRS,
CELLULAR_ACCESS_TECHNOLOGY_UMTS,
CELLULAR_ACCESS_TECHNOLOGY_EDGE,
CELLULAR_ACCESS_TECHNOLOGY_LTE,
CELLULAR_ACCESS_TECHNOLOGY_LTE_CAT_M1,
CELLULAR_ACCESS_TECHNOLOGY_LTE_CAT_M2,
CELLULAR_ACCESS_TECHNOLOGY_NB_IOT,
};

As you can see, only LTE alongside the other LTE variants that you already cannot use are defined in zephyr/include/zephyr/drivers/cellular.h

https://github.com/zephyrproject-rtos/zephyr/blob/41020104565711a9b8a03a611a14fdcf43b6a5c6/include/zephyr/drivers/cellular.h

[olalonde]

I am not sure what you mean... The enum you copy pasted includes non LTE technologies like GSM. Where do you see that only LTE is supported?

[vytvir]

[00:00:34.519,073] modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x0d because 0x7e was expected.
[00:00:34.519,073] modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x0a because 0x7e was expected.
[00:00:34.519,104] modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x43 because 0x7e was expected.
[00:00:34.519,134] modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x4f because 0x7e was expected.
[00:00:34.519,134] modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x4e because 0x7e was expected.
[00:00:34.519,165] modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x4e because 0x7e was expected.
[00:00:34.519,165] modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x45 because 0x7e was expected.
[00:00:34.519,195] modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x43 because 0x7e was expected.
[00:00:34.519,195] modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x54 because 0x7e was expected.
[00:00:34.519,226] modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x20 because 0x7e was expected.
[00:00:34.519,226] modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x31 because 0x7e was expected.
[00:00:34.519,256] modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x35 because 0x7e was expected.
[00:00:34.519,256] modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x30 because 0x7e was expected.
[00:00:34.519,287] modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x30 because 0x7e was expected.
[00:00:34.519,287] modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x30 because 0x7e was expected.
[00:00:34.519,317] modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x30 because 0x7e was expected.
[00:00:34.519,317] modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x30 because 0x7e was expected.
[00:00:34.519,348] modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x30 because 0x7e was expected.
[00:00:34.519,348] modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x30 because 0x7e was expected.
[00:00:34.519,378] modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x0d because 0x7e was expected.
[00:00:34.519,409] modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x0a because 0x7e was expected.

As a response to ATD**99***1# you are getting <CR><LF>CONNECT 115200<CR><LF>. This is normal, because for Quectel BG95 we don't expect a response (likely a small nit or due to some nuance) and it immediately starts trying to decode PPP frames (this is why it is expecting 0x7e which is the start indicator for PPP).

Based on +CREG: 1 you are connected to a circuit switched network. Circuit switched networks only support voice and SMS, not data, which is likely the driving factor of why L4 wasn't connected in time. If you can provide more detailed logs, we can dig a bit depper.

To the dial-script commands try adding AT+CGACT? and AT+CGDCONT? after the AT+CFUN=1 command is executed.

[n3rd4i]

To the dial-script commands try adding AT+CGACT? and AT+CGDCONT? after the AT+CFUN=1 command is executed.

Hi. I have the same issue described by this ticket, I would like to report the output of these commands:

[00:00:02.090,000] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_dial_chat_script: step: 2
[00:00:02.090,000] <dbg> modem_chat: modem_chat_script_next: sending: AT+CFUN=1
[00:00:02.170,000] <dbg> modem_chat: modem_chat_log_received_command: OK
[00:00:02.170,000] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_dial_chat_script: step: 3
[00:00:02.170,000] <dbg> modem_chat: modem_chat_script_next: sending: AT+CGACT?
[00:00:02.230,000] <dbg> modem_chat: modem_chat_on_unknown_command_received: +CGACT: 1,0
[00:00:02.230,000] <dbg> modem_chat: modem_chat_log_received_command: OK
[00:00:02.230,000] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_dial_chat_script: step: 4
[00:00:02.230,000] <dbg> modem_chat: modem_chat_script_next: sending: AT+CGDCONT?
[00:00:02.290,000] <dbg> modem_chat: modem_chat_on_unknown_command_received: +CGDCONT: 1,"IP","eciot","0.0.0.0",0,0,0
[00:00:02.290,000] <dbg> modem_chat: modem_chat_log_received_command: OK
[00:00:02.290,000] <dbg> modem_chat: modem_chat_script_next: quectel_bg95_dial_chat_script: step: 5
[00:00:02.290,000] <dbg> modem_chat: modem_chat_script_next: sending: ATD*99***1#
[00:00:02.290,000] <dbg> modem_chat: modem_chat_script_stop: quectel_bg95_dial_chat_script: complete
[00:00:02.350,000] <dbg> modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x0d because 0x7e was expected.
[00:00:02.350,000] <dbg> modem_ppp: modem_ppp_is_byte_expected: Dropping byte 0x0a because 0x7e was expected.

I would also like to help on debugging this issue. regards.