New ZenWifi Pro XT12 router and node cannot connect to each other

[hinochi]

Erstwhile lurker in these forums and have picked up much advice recently, thanks to all your contributions! I have a super frustrating problem I hope you can advise on, 'cos I've had absolutely no traction with Asus customer support.

Current Mesh setup with wireless backhaul (old but works fine):

• RT-AX86U as router
• RT-AC86U as node

New Mesh setup bought on sale because the XT12 2-pack was half price! (both running Firmware version 3.0.0.6.102_3519):

• XT12-1 device as router
• XT12-2 device as node

Gone through the setup:

1. First test: before doing anything, connect PC to each device in turn with ethernet cable, can reach the asus.router page on both devices. Great.
2. Second test: Set XT12-1 device as router, set up the separate three bands SSIDs and passwords. Great. Try to add XT12-2 device wirelessly as node via AI Mesh function in Web GUI, and QIS page. Both devices are 10cm apart. Never connects, can't be seen as an available node in Mesh wizard. Flashing blue LED, device goes into a cycle of various flashing LED colours (white, green pulse, blue, blue flashing). Crash, device restarts, rinse repeat until you turn the device off.
3. Third test: Try to add XT12-2 device as node via wired ethernet cable. The router XT12-1 device never acknowledges the node device is connected. Disconnect ethernet from the router device. Try to connect PC to XT12-2 device via ethernet cable. Cannot now reach Web GUI by asus.router address, or 192.xx on the XT12-2 device. What the hell.
4. Fourth test: use Web GUI on XT12-1 device as router, try to connect old devices RT-AX86U and RT-AC86U as nodes via AI Mesh setup wizard. Success. Immediately. From the basement 35 feet away, and one floor down.
5. Fifth test. Hmm. Maybe XT12-2 device is somehow broken? Hard factory reset both XT12 devices. Do the same setup steps, once via Web GUI and once via ASUS Router App opened with QR codes on the device bases. XT12-2 device now as the router. Exactly the same happens but that XT12-1 device as node cannot connect via wireless or wired ethernet connection.
6. Try to connect PC to XT12-1 device via ethernet cable. Cannot now reach Web GUI by asus.router address, or 192.xx on the XT12-1 device as the node.
7. XT12-2 device as the router is not inherently broken; RT-AX86U and RT-AC86U devices as nodes can connect to it instantly in Mesh setup via Web GUI or ASUS router app.

Completely confused about what to do next. Only one device (the one set as router) ever works.

As a single router, it's a beast. Already see the improvement in coverage and speed for all my clients compared to the RT-AX86U. But it's a bit pointless having a 2-pack for tri-band with dedicated wireless backhaul if both XT12s can't be used in the same Mesh setup.

ASUS support just recommended with reboot the router(s) and factory reset each device. Totally stumped as to what to try now. Appreciate any help!