Sim 800L stop working after a week

Hello

i have total 300 sim 800l module, last week we had applied on the field now they are not able to connect to the network.

even i had checked all things but it seems perfect (voltage, current, antenna and etc). even module are original but all module are not working after 6 days one by one goes down and gives AT+ CREG (0,3) output. even not able to latch on a network.

we had also tested with/without antenna but same problem was there.

Did you bring back the devices where they worked earlier and test them if they still work at the lab? May be the problem is network in deployed area.
If they are not working after bringing back to the lab also then there must be some other part that’s snapping off. You could replace a failed board module to another fresh board and see if it registers, to eliminate the board fault.

Yes i had checked it but still that issue happens even i had checked microcontroller and also tried to debug multiple way but no network latch problem still exists.

And this is not in single device ,its in more than 225 device same problem even i had verified module isnt fake or refurbished.what can i do?

What functionality are your board doing. Sending data or sms or call etc?
Which network are these deployed on