You will need 1 main Deco unit to connect to the Airtel router, and set that to AP mode. Preferably a wired connection or wireless but close to the Airterl router with line of sight. Then add any more Deco units (secondary units) as needed but ensure that all secondaries are connected to the main and not to another secondary. If you want, you can move the Airtel router to bridge mode and then use your main Deco unit as the router itself (some QOL features are activated for this in the Deco app) - I didn't use this since I prefer the AP mode and also broadcast the Airtel wifi as backup (Since it has a wifi UPS attached for powercuts).

1 hr long SOP

Hi, I've been meaning to get a mesh WiFi too. So all these need to be connected to the Airtel Fiber cable or just one of them? I am currently using a WiFi extender from Netgear and the connection speeds are sub-optimal. Need a weireless option which doesn't slow the speeds

Got the BE25 Mesh System, working great! Details here -

Got a mesh wifi system, the TP-Link BE25, for my 200 Mbps Airtel Xstream Fiber connection. Works pretty well, getting great pings and DL/UL speeds on my wifi devices as well. Speedtest on Tablet/Phone: Reference - Wired ethernet connection to the laptop: