And @Pico as well:
Sadly, this seems to be a continuation of the perennial Wifi issues on MT76, but it was shared by ATH9 platforms like the C7 all the way through 19.07.7, so likely a bug in something common.
Fix is simple, create a script and use cron to run it every couple of minutes (I use 1 minute, as I get loud complaints if down for >60 seconds):
NOTE: sscript is for the C7, asjust the 'wlanx' to match your devices 2.4GHz radio
# ------- call to check 2.4 Wifi ----------
# if channel != 8 then
iw dev wlan0 scan trigger freq 2447 flush
#else
# use ch 3 -> 2422
#--------- end check ----------
Got lazy and that's as far as I got, I lock that AP in on ch 11, so it does not change. Just left comments to remind me I'm poking ch8.
That works great on both the Archer C7 and on an MT621, both running 19.07.7. I assume it will continue to work on 21.x