MC7455 modem stops responding over QMI and serial

[Alex Ballmer]

I have a few MC4755 modems connected using the qmi_wwan driver (on linux 
4.14.171) managed by modemmanager. After some time a few of these modems 
have completely stopped responding to any commands over QMI, not showing 
up in modemmanager. The AT command serial port (/dev/ttyUSB2) and the 
NMEA serial port (/dev/ttyUSB1) are both missing. The only port that is 
available is /dev/ttyUSB0, which seems to be for diagnostic monitoring. 
Nothing is sent over this port, however.

This issue persists after a cold reboot. The qmi_wwan driver doesn’t 
show anything strange in dmesg log. What could be causing this issue? 
Thanks in advance.