Linux system cannot use the data connection

[Filip Kubicz]

Hi!

I'm using ModemManager 1.18.6 with Quectel EG21-G modem and Twilio SuperSIM.

This setup works ok on multiple devices.
However, on a few devices, I see following situation:

   -

   Modem registered to operator network (e.g. Vodafone UK)
   -

   Modem reports connected state
   -

   wwan0 interface has IPv4 address
   -

   Network provider sees no issues
   -

   in Network provider console you can see the device exchanges a few
   hundred kB of data every hour
   -

   but ping doesn’t work
   -

   sending data fails

This looks strange, as if ModemManager was not aware of the data session
that is setup, or Linux system not being able to use it.

The simplified flow of using the modem:
- enable modem
- wait until it auto-registers
- create a bearer and connect
- when bearer obtains IP address, configure the network interface

I attached relevant mmcli information from the device (modem, bearer) and
debug level logs from 2 situations:
- device stable in connected state (but ping fails)
- modem reset - sequence of enable, register, connect -> goes into the same
state where ping does not work.
I also attached the commands that are used for setting up the network
interface when a new IP address is obtained.

What should I check, or do you know how to avoid this situation?

Kind regards,
Filip
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