I replaced my router from the 2017 version to the latest  2021 version. It has been provided by my ISP itself (Jio Fiber). 
After the change of routers, the extender just won't connect to the WiFi. I'm even using 2.4GHz. I have reset it almost 11 times now, tried disabling Client Access Control, tried disabling the firewall but nothing works. 

I tried setting up a mobile hotspot for the range extender to extend, and that works just fine. This means that the problem is with my new router.

The RE light doesn't switch on while setting up either. All I get in the app is that the extender could not connect to the main WiFi thus, reselect the network. 
I didn't have any such problem before replacing the router. 

Please feel free to be as technical as you want, I don't mind.