Cellular wan disconnect or Sim Card not ready on LTE3301 Plus

Runner73

Hi,
from a couple of days I'm experiencing strange behavior on a LTE3301 Plus.
The router is configured using cellular line for the WAN.
It works correctly for a 30-45 minutes, after this period without any error in the log loose connection with cellular line or sim not ready message appear. If I reboot all works correctly.
Obviously I tested the sim in a different router and it works perfectly.
I search for new firmware but on the zyxelsite I found only the original one.
Is it possible an hardware failure or someone could suggest me detailed test to identify the problem and how to solve?
Regards