What you report[1] could essentially be due to:
- improper configuration
- RouterOS needing an upgrade (or a downgrade)
- firmware of the modem needing an upgrade
Let's start with #1, follow instructions here and post your current configuration:
Forum rules - #5 by gigabyte091
[1] which doesn't sound at all "normal", in any case there should be no need to reboot, at the most disabling/re-enabling the LTE interface should do, but certainly not every 3-4 hours