Since you have tried all safe steps listed in your original post here is 2 things you can do. Firstly, try using your friends wifi with your phone and check if it gets disconnected. Secondly if issue is still there then visit Service Center. Nothing else will resolve your problem.
tried with different wifi networks, tried with different routers (tp link and Cisco), also tried with different mobile hotspots (airtel, Vodafone and jio) and at the end also tried with Jio wifi dongle. For all its same.
I have posted video as in one comment regarding different wifi network.
I will visit service center. But in this covid situation service centers are open?