Mudi e750 - sim card not registered

Hi, I have a problem with GL-Mudi - e750 with EP06-E Module. The sim card worked for a month without problems but now the modem says: sim card not registered. I tried the sim card in my phone and USB modem and it works perfectly fine. Apn is also set correctly.

Router Mudi e750
Modem: EP06-E
Firmware: 3.217

I tried:
Modem reset
IMEI change

Without success

Any idea what to do ?

Reset router to factory default settings

Resetting to factory didn’t help. I also tried another sim card from the same and another operator and the same problem Sim card not registered. I tried the new sim in the phone and USB modem its works

Firmware Version: 4.3.8 stable for Mudi is available now to download