If you have the nodes connected over a managed switch "Ethernet Backhaul Mode" in the System Settings, it will fail always . I had that experience already.Yes, Ethernet Backhaul Mode is checked and always has been. Ok, reset the router and set it up again. Reset one of the nodes and added it back in again. It gets added with a weak wireless backhaul despite Ethernet backhaul being checked, stays connected for about a minute, and then disconnects entirely.
While I had set "Ethernet first" within the nodes it worked and and displayed Ethernet as Backhaul. But with this setup there was still a 5Gig Wifi Band reserved for Backhaul.
I connectect then the nodes directly at the Routers Ethernet Port and Ethernet Backhaul Mode works no without any problems. Try to find an unmanaged Switch if you can't connect the nodes directly. Maybe someone watching this thread may give a hint if some managed switches will work as well.