I think this has something to do with the airtel connection itself in yours area... Have you checked with airtel? This is usually the case with the network.. Not the device
If you are a M series user then get used to it. I also face the same issue with my Airtel SIM. Network provider is not always responsible, sometimes mobile is also responsible for some network related problems. Because it's the phone that is going to recieve the signal, so if the receiver is not so good at doing its job then network provider can't do anything. I have complaint about it many a times with Airtel and checked with other devices but never faced this with my ASUS Phone. You can try an app named Force LTE only (https://play.google.com/store/apps/details?id=com.xsquarestudio.forcelte) to select 'LTE only' but you will face frequent signal issues.
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