MT7986 - MT6000 - 5GHz WiFi freezes - DHCP responses but no more internet connection

Subject of the issue

I use the MT6000 with OpenWRT snapshot r26084 as a public WiFi router with a high volume of clients. After some time the 5GHz WiFi is unusable / freezes.

You are still able to connect to the network, receive an IP via DHCP, but no internet traffic is going through anymore.

The connection to the 2,4 GHz network works without problems. There are no errors in the syslog/kernellog except the usual "Invalid band" error message.

https://github.com/openwrt/openwrt/blob/v23.05.3/package/kernel/mac80211/patches/subsys/780-avoid-crashing-missing-band.patch

Environment

OpenWRT snapshot r26084 + wireguard
Public WiFi
2,4 GHz and 5GHz SSID are the same + MultiSSID
5GHz AX and 160 MHz

Steps to reproduce

OpenWRT snapshot r26084, public 5GHz WiFi with a high client volume. SSID on 2,4 and 5GHz are the same + MultiSSID. Wireguard (optional). WED is disabled, only software acceleration is activated. AX + 160 MHZ, AP isolation

Expected behaviour

5GHz running stable

Actual behaviour

After some time it is not possible to connect to the 5GHz channel anymore. DHCP requests are answered but no internet traffic is going through. Even reconnecting several times to the 5GHz WiFi network does not fix the problem. The only solution is to restart the router or to restart the 5G WiFI interface until the problem will show up again. This problem occured so far on all OpenWRT snapshot versions before.

Start with the reported MT6000/OpenWrt snapshot setup and the mac80211 patch at package/kernel/mac80211/patches/subsys/780-avoid-crashing-missing-band.patch. Reproduce the high-client-volume 5GHz AX/160 MHz configuration and inspect the syslog/kernel log around the Invalid band message. Done means 5GHz remains usable for internet traffic without restarting the router or WiFi interface.