Hi @larsjeh,
Most possibly some TCP header parameters that are different in the packets sent from the host OS and the containers are causing the modem to trip. To be able to narrow down the issue a packet capture needs to be run and packets to be checked with wireshark. If you still experience the issue I may help out with running the diagnostics for you.
Some other important aspects are to run latest firmware on the modem, and also make sure you run the latest balenaOS released as it contains an updated ModemManager version.
Thanks a lot,
Zahari