Airtel reserves bandwidth for postpaid users, end result is that they get much better reception. But then again, if the modem is of poor quality, it may not make much difference. Give it a try for one number by taking cheapest plan they have. Switch to 4G and see if that helps as 4G modems have pretty much peaked in efficiency and atleast in cities, 4G bands are now little less crowded.
This happened in office:
I had iPhone 13 and my colleague had Vivo X80. I am on prepaid and he is on postpaid (both on Airtel). There are areas around office where it takes an effort to make upi scan/payments for me while he rarely faced issues. He used to say that it is because he is on postpaid. Once I moved to iPhone 15 pro max, I am getting reception even in elevator and am not facing any issues with upi payments or calls in those same areas. This has to do with much powerful X70 modem from Qualcomm. Phones with 8 gen 3 will have even better reception. Lately network recpetion deteriorated where he is not able to make UPI payments while I do not face any issues 
