@AnnoyedUser69 

It appears wired link between your switch and Main Deco is broken. Network layout will be the following and it explains what you see:

Node 2 can't connect by Ethernet to Main Deco, so it establishes wireless backhaul link. Everything wired to switch passes Internet traffic through Node 2. Because of that, every device connected to the switch is shown in Deco app as connected to Node 2. 

This theory is easy to test: do nothing else but power off Node 2. If I am right, all devices wired to the switch will lose Internet after that, which will prove there is no wired link between the switch and Main Deco.

  @Alexandre. Thanks for the suggestion. So I powered off node 2 and that did indeed break everything. I unplugged and replugged the switch ethernet cable and then everything came back online (and appeared correctly under node 1). After that I have since turned on node 2 and that is also displaying correctly (wired connection to node 1 and no connection to wired devices). I am monitoring the network now as the node 1 to node 2 connection has appeared correctly (wired) earlier this week, but it then "broke" leading to this original post. Do you know of any diagnostic tools to test my wired connections?

  @AnnoyedUser69 

There is no simple diagnostic tool I can recommend.

Your switch reports link connection and speed for each port, by LED lights on switch ports. 

This should be enough to tell if link is broken.

  @Alexandre. Would you expect intermittent issues if there was a fault with a wired connection?