When using biktor's alternative modem firmware, you may find the modem disappearing after waking up from suspend.
To get rid of this problem persistently, the udev rules from eg25-manager need to be adjusted as noted in SETTINGS.md. If you run into this problem, it's recommended to adjust the udev rules manually for now as described there.
Besides that, you can bring back the modem once by restarting eg25-manager:
$ sudo service eg25-manager restart
Adjusting the udev rules upstream is being discussed in eg25-manager!49.
UPDATE 2022-06-17: fixed in pma!3235, pma!3240