I can confirm I'm having the same issue with a Samsung Note 10+ and a WRT1900AC, running OpenWrt 21.02.0.
The phone is connected, until I start doing something, like watching youtube. It then looses connection (wifi icon with exclamation mark). If I don't do anything, it sits there for about 5 minutes, then it gets a connection again. I can manually disconnect and reconnect to fix it as well, but it's super annoying doing this several times per hour.
Logs don't say much, except for these lines, of which there are plenty, also for other devices:
Tue Oct 5 14:01:31 2021 kern.debug kernel: [428683.276860] ieee80211 phy1: Mac80211 start BA d6:ff:e1:bd:65:ba
Tue Oct 5 14:01:31 2021 kern.debug kernel: [428683.333366] ieee80211 phy1: Mac80211 start BA d6:ff:e1:bd:65:ba
Tue Oct 5 14:01:31 2021 kern.debug kernel: [428683.388916] ieee80211 phy1: Mac80211 start BA d6:ff:e1:bd:65:ba
Tue Oct 5 14:01:31 2021 kern.debug kernel: [428683.448859] ieee80211 phy1: Mac80211 start BA d6:ff:e1:bd:65:ba
Tue Oct 5 14:01:43 2021 kern.debug kernel: [428695.333484] ieee80211 phy1: Mac80211 start BA d6:ff:e1:bd:65:ba
There is no new entry when the phone finally gets internet again.
I was led to believe these lines are not problematic.
I have tried disabling "Allow AP mode to disconnect STAs based on low ACK condition", but that doesn't improve things.