Software update to 12.4

See if this offers any help: https://support.apple.com/en-us/HT204027

Yes, this sim did not work in my other phone. However, when I inserted another sim to my handset it worked perfectly. So it was obviously the problem with the airtel sim. Since my sim was working fine until I upgraded the OS to 12.4, I think the new update did something to the sim and it stopped working

Have you tested your SIM card in other devices? Does it work?

You get cell service from your provider and they are the only people that can help you.

Since my service was down, I bought a new sim from airtel and now my services are up!

Yes, after I updated to iOS 12.4 today, I am unable to access my mobile netwrok. Airtel says that it is my system problem and not network issue. I have restarted my phone twice and reinserted the sim card once. Nothing works

No.

Nothing to do with the update to 12.4.

yes. my iPhone stopped getting connected to network

i called airtel and they confirmed that issue is at the instrument level

What the heck does that mean?

It's their problem and they are trying to con you.