@videoarun 

Hi, do you mean the MR6400 doesn't receive any 3G/4G signal now by using Airtel SIM card? and what is the network type displayed on Advanced->Status page?

In addition, please insert the Airtel SIM card into another 4G router or smartphone, then check if internet works fine and also allows for sharing internet(test phone hotspot), if so, please check which 4G band the other router or phone is using at the same location. Here is the instruction about how to check 4G band on smartphone:

https://www.tp-link.com/en/support/faq/3488/

If the issue only happens with the MR6400, please send me a system log file as well.