I've been experiencing the same issue over the last couple of months. I've renamed the 2G SSID but I'm getting Ubuntu, Android and iOS devices all being disconnected from 5G and unable to reconnect - even though they temporarily appear in the 5G station list while they try. If I reboot or stop/start 5G they will connect fine. It can stay up for a couple of weeks, or 10 minutes before dropping connections again. 2G is fine at the same time.

From when I bought it in Feb 2023 until the last couple of months it worked perfectly, so maybe a recent firmware upgrade has caused the issue? I'm on 4.4.5.2_BT

I'm experiencing something similar but different. Posting it here to see if anyone can offer me any advice or troubleshooting tips.
Got my 2866ax a week ago. Upgraded not to 4.4.5.2_BT and set it up. Don't think I changed much from default choices. All seemed fine.
Within a couple of hours the wireless died. The only thing to fix it was a reboot.
Since then I've found this thread...so I've tried:

• Separating 2.4GHz and 5GHz
• Disabling 2.4GHz
• Disabling 5GHz

All have the same result - within a few hours everything wireless stops.
The lights on the router indicate that everything is fine.
On the client side the behaviour varies:

• No new devices can connect
• Existing devices may or may not appear to be connected (but none of them are, everything just times out - ping, http, etc.)

My concern now is whether I just have a bad box and a new one will fix it...or if I'm experiencing a firmware bug.
Any hints or tips much appreciated.

I put something similar to this reply on another post I think - but -

Do you have a Sky Q setup in the same property? - if so have a good look at how that's setup as Sky's networking leaves a lot to be desired.

I have just upgraded from a 2925AC to a 2856AC in advance of an increased speed FTTP which the 2925AC will not support.

The 2925AC was solid on Wifi once I set 5ghz to band 36 so DFC did not happen. So for the past few years its been running fine with the only issue being the 5ghz does not like walls.

I couldn't do a direct settings transfer between the 2925 and the 2865 but was able to move the items such as ip bind listing etc. I then sat on two PCs connected to the 2 routers and went page by page and set them up identically where possible.

The wifi performance is worse on the 2865, random lockouts only cured by connecting to another wifi SSID and then back again, also sometimes not able to connect to some SSID's at all.

I am on firmware 4.4.5.2_BT so am going to try 4.4.5.3 RC3 to see if it improves, if not then the drill is coming out and the network cables are being installed.

No SKY to interfere and security set to WPA2/PSK.

I don't think this is particularly linked to your issue but Draytek did appear to use a poor (probably cheaper) wifi chipset in some routers. Amongst other things it prevents you from using wideer channels on 5GHz band if wide channels aren't also used on the 2.4GHz band...the 'ax' parts aren't affected. The below was just one of the responses I got when trying to 'fix' this (so if you have any chance of changing to an 'ax' model you probably should anyway!):-

There has been no further progress on resolving your issue.

The issue is caused by a known hardware limitation with the Wi-Fi AC chipset (2135ac/2765ac/2927ac/2865ac/2866ac),
This cannot be resolved by updating firmware.

They mentioned that 5 GHz can work at 80 MHz when 2.4 GHz is disabled.
Also, "5GHz will not work on higher channel bandwidth unless 2.4 is set to 20/40MHz or 40MHz."

Well after an afternoon/evening of lockouts for a few minutes every hour or so I gave up and installed an Linksys AP next to the router and used its Wifi 6. Two days with no issues and the router has not had to be rebooted.