Ideally a 2866Vac, but any would probably do. If you can help, can you try this...
Disable both 2.4 & 5GHz WiFis.
Enable only the 5GHz, and with Channel=Auto
Try to connect to the 5GHz-only SSID. Preferably with an iOs or iPadOS client.

Does it connect?
It doesn't for me. The 'connecting...' wheel spins forever, never connecting.

If 2.4GHz is also active, no problem, I can connect to 5GHz.
If Channel is set to /anything/ manually, no problem.
But 5GHz-only, and Channel=Auto.... nothing can connect.

Thinking it was a hardware fault, Draytek kindly issued an RMA. I returned the unit, they confirmed they could reproduce the fault, so the replaced the PCB and returned the unit to me.
Problem is still there!
So wondering if it could be a bug that affects all 2866's. I borrowed a 2766Vac and that does NOT exhibit the problem..
Thanks for your help

Mark
 

I bet it affects the 'ac' units only (and probably all the other 'ac' units of that generation) - that chipset is fundamentally flawed (I've posted about it's weird limitations before - and had them confirmed by Draytek - the earlier models 2860/62 are actually more capable!).