OK, I will post this here for the benefit of other poor sods who may find themselves Googling here for a solution. I have spent all day on this, but FINALLY managed to get it all working.
I tried everything to get this to work:
Updated the new XT8 node firmware
Loaded factory defaults
Reset by holding down WPS and powering up
Reset by pressing the reset pin for 5 seconds
Tried the Asus Router app
Tried the Asus Router app on other devices
Connected to the node via WIFI
Connected to the node via Ethernet
Rebooted everything MANY times
Tried adding a node via the startup wizard
Tried adding a node via the web UI
Disconnected EVERyTHING
Tried it all again many times
Nothing works. Apart from this:
1. Connect the node to the main unit via Ethernet - yellow ports on the router, to the blue port on the node.
2. Log in to the main router UI and select Network Map on the left
3. At the bottom of the screen in the middle, click on AI Mesh Node
4. On the right hand side, click on Search
5. When the new node is found, click on Yes (or attach or whatever it says - I cannot quite remember)
This SHOULD work. You should now have a node connected with wired backhaul over Ethernet.
6. Now click on AIMesh on the left
7. Click on your new node in the middle of the screen
8. On the right hand side of the screen, click on Management
9. Change Connection Priority to Auto
10. Click on Reboot Node
11. Whilst rebooting, pull out the ethernet cable
Et Voila! You now have a new node connected wireless into your Mesh network.
I am pretty angry with Asus about this to be honest. It is utterly ridiculous that none of the documented ways of connected a new node should actually work. I could understand it (a bit) if we were talking 3 or 4 different types of hardware, all with their own different firmware versions. But i have 4 x XT8's all running latest firmware and all reset and rebooted and they just will not talk to each other as they are supposed to. A very poor show.