@witchypoo 

Deco mesh runs its own WiFi network. It will not share or join or extend service provider WiFi. What you have now is same as having two independent WiFi routers, one upstairs and one downstairs.

It is recommended, if you want Deco mesh, to have multiple Deco units and also to turn off service provider WiFi.

  @Alexandre. 

So what I need is ANOTHER mesh, connected to the main router (downstairs) and then OFF the main service provider
Then the UPSTAIRS mesh can be wireless or ethernet
But will this allow me to walk all over the 2 x rooms WITHOUT keep reconnecting the wifi?

  @witchypoo 

Using E4 in your setup as an example.

You have configured one Deco E4. It is Main Deco, by default. Take it downstairs to where main hub is and connect to main hub with short Ethernet cable. Add second E4 to your Deco mesh, through Deco app. This will be your Satellite Deco.

Connect second Ethernet port of Main Deco to Ethernet cable that goes upstairs. Take Satellite Deco upstairs and connect it to the second end of that same Ethernet cable.

Turn off main hub WiFi.

With that setup, you can walk between rooms without need to reconnect WiFi.