Its funny as exactly same issues I am having on 2866AX. even 2.4ghz cant work properly, older draytek router worked fine with no problems. There is something wrong with this new range wonder if cost saving exercise been done on wifi chips....
Could you share you settings for both 2.5 and 5ghz.

Come here to have a look at a problem I am having and come across this thread.

I have just put a 2866ax in and everything works fine for a few days and then for no apparent reason the wifi networks just disappear, pressing the wifi on/off button makes no difference and the LAN ports are still working fine.

The only way I can fix it is to turn the router off and back on and they will all reappear for another couple of days.

I updated the firmware before installing although cant tell you the specific version right now.

Anyone shed any light on the problem?

I still haven't saved up enough money nor researched a router/VLAN/mesh system to replace DrayTek and still got poor range and multiple laptops not seeing the 5GHz SSID to connect to from time to time. CCTV reporting poor connection on 2.4GHz guest network. I've just updated to latest 2927ax but there's nothing in the release notes to suggest it'll help. I even tried ANT-1207 7dBi High-Gain Dual-Band Aerials I got off eBay. To be honest I've given up and hence not returned to this forum for long time but someone was asking for my settings, so:
https://ibb.co/k49x6Pg
https://ibb.co/0YQvk3B
(Image embedding didn't work so hopefully above links will)

Hi,

Not that it helps, but I am also experiencing the same wfi dropouts.

Have you managed to find a remedy for your issues ?

Tks.

The 5GHz band will disappear when there's radar detected on DFS channels. This is normal because radar signal takes priority. This is explained here: https://www.draytek.co.uk/support/guides/the-5ghz-wireless-band

Not the cause in my case. My 5GHz is set to channel 44 (band A-Lower, no DFS required).
Even my CCTV on 2.4GHz guest Wi-Fi has poor connection and I don't live in a mansion!
One of our laptops regularly doesn't identify either of our 5GHz SSIDs, although it does see the 2.4GHz ones, but I suspect that's a Windows 11 upgrade/network driver issue.
Getting fed up with technology, it was supposed to save us time lol.