A couple of things.
Is BOTH the routerOS version Current as well as the HapAX hardware board Firmware(System->RouterBoard { }, if not ->upgrade+reboot).
Then after a reboot and the LTE comes back, have you checked the firmware upgrade for the LTE card itself ?
( Ps you could add in a watchdog service, and have the unit reboot in the short term ( if LTE does down ). Only a couple of lines of script.
Also, your not the only person with the issue, but likely firmware related :