Well, I persevered with the AX92u for a little longer.. factory reset everything, flashed the 41712 firmware on the AX92u's and the Ax58u has 41700 firmware. Used the new Ax92u as the router with the other one set to aiMesh mode. Likewise on the Ax58u as well.
Ran this config for a few days. LOADS of wifi dropouts. LOADS of speed issues (mostly wifi) Strange things like IP cameras (wifi) dropping signals then completely unable to rejoin the network.. I had to factory reset them and start afresh.. they then seems to work for a day or so.. then crashed again. Phones / tablets losing internet connection randomly and not being able to rejoin the network at all, even after resetting the client wifi network.
So, with the help of a very good friend, I set about turning features on and off, testing and seeing what if anything gave a more stable network.
Well, today, my router has been up and running for 4 days (I know that isn't great, but considering I couldn't make it past 1 day before, I'm a happy bunny). All IP cameras have remained connected with zero drop outs. "most" clients have remained connected apart from a few random drops outs when moving between mesh nodes..but the big difference now with my new settings, I can reset the client wifi connection on the client itself and it rejoins. I've had zero speed issues with either wifi or wired connections. long may it continue. What did we do?
FIRSTLY - BACKUP.. Just in case.
1. Within wireless / professional we switched off all channels options for Beamforming (including the backhaul which seemed to take an age to do but eventually worked)
2. I only used the 2.4ghz channel for clients. (there are now a few on 5ghz channel 1 and they also seem stable so far)
3. QOS is switched off
4. I bound the IP cameras to the strongest mesh point
5. Extended my lease to clients to initially 5 days, now 21 days. Lan / DHCP Server / Lease time (5 days is 4320000 and 21 days is 1814400)
6. Finally I ran the optimization within the router. Aimesh / optimization
Thats it. I've no idea if I've crippled my chances of getting huge network speeds.. but honestly, i don't care... What I have achieved is a stable (ish) network so I don't get moans from everyone..
PLEASE NOTE: I'm not saying this is a solution for everyone. But it certainly has been the solution for me. If you are at your wits-end.. then have you got anything to lose by giving it a go.. I know I hadn't..
Good luck...