@David-TP  Hi David. I followed and followed the screen directions on my iphone when setting up the decos, I think initially I had a bad connector on the long cable, found that after I got a tester,  but then still had trouble over and over and over, removed cable from the floor joists where the electrical wires run through them, ( just to see if any interferance from the electric stuff, cable is Cat5E). Still not luck. I spend all day and a lot of yesterday on this. I can only conclude that the Deco doesn't like the long cable connected into the bottom port ( 2 gig) cause when I tried it in the other 2 pots it finally connected. If I know now what these Decos are and how they connect, I would not have bought them. I was assuming that they were connect wirelessly like my TP-Link range extender does, but would be all on 1 network.  Very disappointed in them. 

  @DerekT 

Cat5E is not going to be enough to support 2.5 gigabit speeds. To get 2.5 gigabit speed, you need a Cat6 cable.

That would explain why you can use this cable with gigabit Ethernet port, but not with 2.5 gigabit port.

DerekT wrote

If I know now what these Decos are and how they connect, I would not have bought them. I was assuming that they were connect wirelessly like my TP-Link range extender does, but would be all on 1 network. 

  @DerekT 

Satellite Deco should connect wirelessly if you do not hardwire them and they are in WiFi range from Main Deco.

All Deco nodes will obviously be on the same network if you setup them properly, because this is the core benefit of WiFi mesh: one SSID, multiple WiFi nodes with seamless roaming between them.