@knownoname It seems what you're describing as a goal is the Easy Mesh with Ethernet Backhaul itself. If you unplug the cable of one or both AX55, Easymesh should switch/revert to a wireless backhaul...

@BunzZz What's your main router LAN IP address and what's the WR840n LAN IP address ? In general these LAN settings should work out of the box.

@Abhi2704 You can always search for a third party FW but I doubt that.

@Abhi2704 Using WDS on w8961n is different that other devices and you may not be able to use it with another device than w8961n - details.

@BunzZz Here's the guide for configuring a router in AP mode (Case 1). It's mandatory that the LAN IP address of the device in AP mode to be in the same subnet as the main router. i.e: If the main...

@cashchow In TP-Link's terminology that's called WDS/RE mode. Unfortunatelly C3150 doesn't support this feature - here's the list of router supporting this feature. And here's how to configure if...