Not all Airtel users are behind CGNAT. Don't know about @prime but few of us including me still get a public IP.Bridge mode is barely having any advantage now with Airtel Fiber (unless you have a static IP) since it's behind a CGNAT anyway.
This has a major disadvantage. It will put him behind a double NAT.You can just connect both routers LAN to LAN port (have your IP of TP-Link set to in the same subnet as that of Airtel) and disable DHCP server of Airtel router and enable the same in TP-Link router. Make sure to set the Gateway IP in TP-Link to that of Airtel Router and configure DNS as you want and then restart both routers.
It's a quick and dirty way without bridge mode and has no disadvantages.