Hey,
Just had this problem this morning. Suddenly my LHGG LTE6 kit had no internet connection. When i looked at the status page of the lte1 interface it said that "sim card not inserted".
I managed to fix the issue by keeping the router powered on and looking at the lte1 interface via winbox and inserting and removing the sim card. Had to do it multiple times when finally PIN status was "ok".
This was very weird because i attached the sim card once while setting the LHGG up and did not touch it at all for about 8 months (until today).
Reading through old posts i see that this is an ongoing issue with a lot of mikrotik products. The sim card slot is a bit faulty.
This i just a PSA if anyone runs into the same issue.