On my 2866Vac, if I set Channel to Auto, neither my iPhone 13 not my 2024 model iPad can connect.
They try to connect, but never succeed (the spinning wheel icon spins indefinitely).
BUT if I Disable Wireless LAN, pause, Enable it and on the same Channel that AUTO was using, both devices connect within a second.
[loop]
If I disable again, set to Auto, neither device can connect.
This is reproducable 100%.

At first I thought it must be DFS-related, but the same failure occurs even in Auto selects an A-Lower channel.

Any thoughts or siggestions appreciated (except for Don't use Auto, which i already thought of!).

I have all sorts of issues with Wifi and failure to connect on firmware 4.5 on my 2865. If I go back to firmware 4.4.11 everything can connect fine.

That's odd, because on my 2866ax there is no "Auto" option at all in the Channel drop down under the "Wireless LAN (5GHz) -> General Setup" page. Almost like they knew!

Is yours an ac or ax model I wonder?

2866Vac

Ah yes, I feared as much, that ac wireless chipset is fatally flawed in many ways, some acknowledged by Draytek, some not. e.g. Have you ever tried getting IPv6 to work for wireless Android devices? My advice is don't bother, because it doesn't work period!