@DB860 

One Deco unit will need to be wired to the router. You can have the network and password be the same as your existing router, but it would be recommended to disable the Wi-Fi on your MR600 to allow the Deco Wi-Fi to provide the mesh network. 

@Tony thanks for the response. Ended up setting up a new network name so I could keep the functionality of all the Deco app features. I believe setting it up in AP mode on the Router network would have meant my devices didn't need to be relinked to the network but all control would then need to be through the Tether app.