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.