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I note that you mentioned the modem has a USB interface?
If that’s the case, then that could be your culprit. I’ve found that anything USB can be problematic, particularly on a computer that is running continuously over a relatively long period of time. I’ve gotten into the habit of rebooting every day, if possible (a hard reboot, switching all devices off, particularly USB, and then restarting the USB devices once the computer has rebooted).
Power issues are the more likely cause, particularly if these issues have just started, but I’d just be aware of potential USB considerations as well.