@Kranthi-Swaroop If your WR840N is HW v5 use these FW releases. If you use a wrong HW version / FW region file you'll receive this kind of error. HW version and FW region is printed on the bottom...

@waggy75 Set VR400 LAN IP address to be in your Deco subnet and disable VR400 DHCP server: Disable VR400 Wi-Fi radio (both bands): Now you can use VR400 like a plain switch (ports from LAN1 to LAN4).

@Zebop917 Here's a guide on how to configure port forwarding on VR400.

@WMC When using the WDS feature both routers' LAN IP address should be on the same subnet i.e. 192.168.1.0/24. Follow the WDS configurations guide then test if everything's OK.

@CathalFerris I assume you're referring the Address Reservation limitation of 32 entries only. For the DNS setup - that's by TP-Link's design - refer this thread for more details. Here's the official...

@danielrosehill Refer this thread.