MC7455 modem stops responding over QMI and serial

I have a few MC7455 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.

I guess the module has been crashed and entered in to Qualcomm download mode.
Do you think you can put the module in Windows and see when is displaying in device manager?