It seems to now be working at least a bit better. Earlier I needed to boot it every 2 hours but now it's running already for 8 hours.
What I did:
1.Downloaded the latest modem firmware RM520NGLAAR01A08M4G_01.206.01.206.
2. Renamed the file name from RM520NGLAAR01A08M4G_01.206.01.206.zip.zip to RM520NGLAAR01A08M4G_01.206.01.206.zip.
3. Opened System -> Upgrade -> Module local upgrade and drag’n’dropped the zip file there and ran the upgrade.
4. Removed 2nd SIM card (which was not even used).
5. Disabled Network Acceleration under Network tab.
Would someone know if there's a better way to reload the modem than just rebooting?
Eg if-down and if-up or something else?
Currently I'm checking if the modem stops responding with this command:
/usr/bin/timeout 30s /usr/bin/gl_modem -B 0001:01:00.0 AT AT
And then doing the reboot but this takes quite long time and I'm wondering if there's something else I could do to just reload the modem without booting 
.