Adding XT8 to Mesh with ethernet = blue led

[pvanosta]

I have a few ethernet jacks throughout my home (2 on the ground floor, 2 on the first floor). I have a Mesh of XT8's dotted around the house. The two which are on the ground floor, are connected to the ethernet LAN and ultimately via a switch to a Fiber modem.
I have individually tested the ether ports by walking around with a laptop and connecting it directly to each ethernet port. I have 1GB down and 1GB up on every port in this way (limitation of the laptop).

I have added an XT8 to the mesh by putting it next to the master and connecting the WAN port to the switch. All is well. White led => good to go.
I take it upstairs and plug it into the ethernet port there (which I already know gets GB signal) and power it up. Blinking blue LED.

Side note: I still get the speed benefit in that room, both via wifi and via ethernet to one of the LAN ports. But in the mesh it remains invisible / not connected.

What am I missing / doing wrong?

[pvanosta]

Not quite. Ethernet connection (or the ability to get it going) is the heart of the problem. The main router is connected to the Fiber modem via ethernet and that connection shows up correctly (2 lines) in the diagram. The node in the network closet is connected via ethernet but that connection only shows up as a single line (so I am guessing it does not recognize the ethernet connection as active). If I change the backhaul from wifi to WAN, the system goes kaboom and I get the blinking blue led. If I connect either one of the two upstairs nodes to a (working properly) ethernet port, I get the same result. 

How do I check the hardware versions?

firmware versions are all 3.0.0.4.388_24621, except for 1 xt9 unit, which is on 388_24609

The only thing I can think of tryin g, is to take the whole network down, do a factory reset and rebuild node by node, starting from the network closet this time.

Then adding node by node with ethernet connection to the switch and see if that works without falling over.

The fiber modem has its LAN output connected to a 2.5G switch. Should I connect the WAN port of the XT8 Router  Master to the switch or to the LAN output of the fiber modem?

[pvanosta]

Update (in case this is useful for anyone in a similar situation): I took down the entire mesh and started from scratch:

1. reset node 0 to factory settings and connected it to the 2.5GB switch next to the fiber modem

2. I changed the operating mode from 'Router' to 'access point', thereby removing the second DHCP source which I suspected was gumming up the works

3. One by one, I reset and then re-added nodes via ethernet connection to the same switch, then moved each node to its final location (office, bedroom, ...), ensuring each node retained connectivity via ethernet as I did so

4. added one final node via wifi 

Result: stable network with full speed across the house.

Thanks to all who helped troubleshoot the issue 🙂