@hariks No, you can't. Only through wireless connection between those devices - see OneMesh FAQ - Q5/A5.

@Baris Here's a guide on how to configure C6 in AP mode. Keep in mind that C6 LAN IP address should be in the same subnet as your main router. Setting the C6 LAN IP to Smart IP(DHCP) should be fine....

@H.B You can upgrade your VR400 v3 FW manually downloading the FW from here. Here's a guide on how to upgrade locally.

@ZS1YT In both cases of router mode or AP mode the Deco SSIDs can be configured to be the same as your C80 SSID.

@Sethiest You should be able to do the connection in question as described in the TP-Link guide. You may also read through this guide and pay attention to the red notes for different scenarios.

@ZS1YT OneMesh and Deco mesh are different technologies and they don't work together. What you could do is to use your C80 as a main router and configure the Deco E4 in AP mode. Here's more info on...