NEW MTNL Postpaid SIM SIM800A/900A/808 problem

Hello friends,
I recently got a NEW postpaid MTNL SIM. I want to make SMS receiving station. When I put the SIM in SIM800A/900A/808 module, it connects to tower for first few minutes and receives SMS and calls. But after sometime it doesnt receives SMS and says “out of network coverage” when calling. Why is it so ? Its happening in all the three 800A, 900A/808 GSM modems. The same SIM works fine in my MotoG5 phone. However my 10 years old MTNL prepaid cards work fine 24x7 perfectly. Shall I got for SIM7600. I need a solution it because my project would be on for 24x7. Any advises or points to look out for.