Do you remember if it was a "proper sized" SIM or a smaller one with an adapter?
There are reports (not only on Mikrotik hardware) of issues with the latter, see:
Device got hacked 1 min after connected to internet - #14 by jaclaz
Device got hacked 1 min after connected to internet - #23 by jaclaz
but it depends on type/model (and probably manufacturer) of the actual SIM socket, I have seen an installer of elevators/alarms that puts a bit of electrician sticky tape on the back on the SIM and then "forces" it in the slot, but there is the risk of bending the pins.