@ferus 

It should be easy to test your theory on limit for Satellites with Ethernet backhaul: power off "Sam's room" Deco and if what you think is correct "Living Room" Deco, the only one Satellite left, should switch to Ethernet backhaul.

I doubt this will happen, but you should run this test to prove or disprove your theory.

I still suspect Ethernet cable between "Living Room" and Archer C5. Take "Living Room" Deco with that Ethernet cable, bring it to where "Sam's room" Deco is and connect "Living Room" Deco to available Ethernet port of "Sam's room" Deco using that same Ethernet cable.

If everything is OK with Ethernet cable, you'll see in Deco app "Living Room" Deco connected to  "Sam's room" Deco with Ethernet backhaul.

  @Alexandre. 

Hi, thanks for the reply. 

I tried to power off Sam's Deco and reboot the Living room one but (just as you suspected) nothing has changed in the Living room - it did not change to ethernet.

It's a good idea with the cables but mine are built in the walls. 

I did suspect that this could be the issue so I tried the same cable which I use for the Living room deco and connected my laptop to it - the ethernet connection works fine.

All I can do really here is to swap the Deco units but not the cables.

I'm not sure what's wrong right now.

  @ferus 

Swap Deco units - that is the good idea.

You can still try test I suggested with connecting one Satellite to another, but use short Ethernet cable. This is to confirm Ethernet ports in Deco units have no issues.

  @Alexandre. 

I've done the following:

Power cycled all 3 Decos - no change.

Swapped the 2 secondary Decos (Living room and Sam's room) and there was no change either (the Living room ethernet was still giving me trouble). 

Updated the FW to the latest on all 3 Decos - still no change.

Connected the Living room Deco directly to Sam's Room Deco and the ethernet connection appeared in the Deco app.

So... It seems like something is wrong with the Living room ethernet outlet.

BUT when I connect my laptop to it directly, it works perfectly fine. But neither of the 2 secondary Decos are getting a connection it seems.

  @ferus 

I can't offer solution to fix Ethernet cable or to make Satellite Deco use it if it does not want to.

I can think of a workaround you could try. Do you happen to have gigabit networking switch, preferably of TP-Link brand, to test that workaround?

This is how it is going to work:

1. Connect networking switch to Ethernet cable instead of Satellite Deco;

2. Connect laptop to switch to confirm switch did establish Ethernet link (in case it is budget switch without link status LEDs);

3. If laptop has Ethernet/Internet access, connect Satellite Deco to that switch with short Ethernet cable and it should establish Ethernet backhaul.

In case you don't have switch, just get it from the place with return policy, they are not very expensive. You will need TP-Link brand 5-port unmanaged gigabit switch, for example TL-SG1005D.

I used that exact model at my house with Deco mesh, they are compatible. 

  @Alexandre. 

Thanks for all the advice in any case.

I will try it with the switch (it's not expensive) and see what happens. And if it won't work I can always use the switch somewhere else.

In the meantime I found an email contact for support so hopefully they can help me.

But I'm all for any other ideas to troubleshoot this :)