Digi M1 modules fail and cannot read the IMEI

I have a problem with about 10% of my Digi M1 units failing in the field. They cannot contact a TCP/IP server and they cannot even read their own IMEI. Updated of the cellular firmware does not help. They are transmitting every 6 minutes and then they are put to sleep. The power is never removed as they are on Solar charged batteries. Does anyone else have this problem of failing M1 units? If so, has anyone found a solution to these modules failing? It seems that no one that can solve problems is on this forum. Or no one reads these questions. I have tried to contact Digi, but they have given up on helping me. Thanks for any help.