I had seven XBee3 cellular modems quit talking to VZW network all on the same day.
Can you clarify? Does ATAI stop reaching a value of 0? Are you able to reach AI 0, but IP traffic does not work?
If your SIM datapool is paid, I would get one of these modules and see if it still can be reprogrammed to make a Transparent TCP/IP connection to your local static IP. Try to upgrade the firmware and reprogram. I have a field failure problem as well. Some of the modules will never recover. They just die.
If you are using bypass mode, it no longer can make a PPP.