Hi @Stepalex, thanks for your valuable input! After days of frustration, I have finally got it (sort of) working (fingers crossed!).
Here's what I did (in case it might help anyone else)...
Went through the setup process via the ASUS App (not the Web GUI via ethernet cable) for XT12-1 (XT12-2...
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...