zephyr: Problem with PPP + GSM MUX with SIMCOM7600E

Describe the bug I am trying out the GSM PPP driver and GSM MUX with two SIMCOM modems: SIM868 and SIM7600E. It works as a charm on SIM868 but I am having problems with SIM7600E.

When just using MODEM_GSM_PPP=y and GSM_MUX=n it works fine

When enabling GSM_MUX, AT commands works fine and I can run ping in parallel, so network connectivity works to some extent…

My problem is when I start something more demanding on PPP (such as a lwm2m client, or a dns-lookup) the modem halts and then reboots. The same thing happens even if I do not use any AT commands in parallel with the client.

When dumping the logs I do not see anything that is obviously wrong. Is there anyone here who have experience with the GSM MUX and can advise on how to debug this issue?

I have been in contact with the modem vendor and they claim muxing should work in general.

To Reproduce Steps to reproduce the behavior:

Use the LWM2M client sample code

This is the prj.conf I used:

CONFIG_NET_SHELL=y
CONFIG_LOG=y
CONFIG_CONSOLE=y
CONFIG_LOG_DEFAULT_LEVEL=4
CONFIG_LWM2M_LOG_LEVEL_DBG=y
CONFIG_NET_LOG=y

CONFIG_NET_IPV6=n
CONFIG_NET_CONFIG_NEED_IPV4=y
CONFIG_NET_CONNECTION_MANAGER=y
CONFIG_NETWORKING=y

CONFIG_LWM2M=y

CONFIG_MODEM=y
CONFIG_NET_PPP=y
CONFIG_NET_L2_PPP=y
CONFIG_NET_L2_PPP_OPTION_MRU_NEG=y
CONFIG_MODEM_GSM_PPP=y
CONFIG_MODEM_GSM_UART_NAME="UART_1"
CONFIG_MODEM_GSM_APN="m2m.tele2.com"
CONFIG_DEVICE_POWER_MANAGEMENT=y

CONFIG_GSM_MUX=y
CONFIG_MODEM_GSM_SIMCOM=y
CONFIG_MAIN_STACK_SIZE=2048

Impact This is a showstopper for my client

Logs and console output I do not see anything in the logs that looks out of the ordinary. Would appreciate tips on what data to look for.

Environment (please complete the following information):

• OS: Linux, MacOS
• Toolchain: arm-none-eabi
• Commits:

   - name: zephyr
     remote: upstream
     path: zephyr
     revision: 98bcc51b0925a92db28f127d08b842b1d609bce8
     clone-depth: 10
     west-commands: scripts/west-commands.yml
  
   - name: hal_nordic
     revision: 00580f43b9d48f2079d611ce12f0fbc78d3040f1
     path: modules/hal/nordic
  
   - name: cmsis
     revision: 542b2296e6d515b265e25c6b7208e8fea3014f90
     path: modules/hal/cmsis
  
   - name: segger
     revision: 6fcf61606d6012d2c44129edc033f59331e268bc
     path: modules/debug/segger