@Strat640 There is TP-Link AV600 powerline adapter on your network diagram. It should be compatible with Deco mesh, so try with it first. I have ZyXel AV600 powerline adapters, in my tests with them...

@LiorA Do you have your Decos from different geographical region than your phone? That may explain.

@davdav I have Sony SmartTV connected to second Ethernet port of Main Deco, works just fine. Not sure what is going on in your network, but it is unusual.

@cgaertner The final test would be to enable WiFi on TP-Link WR841N, turn off Deco mesh and see if connected to TP-Link WR841N by its WiFi everything works as you expect. If it does, you'll need to...

@cgaertner Yes, Deco mesh will provide WiFi. Think about Deco mesh as an attachment to wired only router and that attachment will be used to provide WiFi signal.

@cgaertner I'll answer on what I can. >(1) Buy another mesh system with more advanced configuration features (e.g. Deco PX50??). Don't bother. Different models of Deco mesh may offer different...