I'm still having problems - random devices seem to be having connectivity issues.
My house alarm just notified me that it's on cellular backup as it lost internet access, then came back online a minute or so later. It uses a wired ethernet connection to my RT-AX88U main router. Looking further I think there are DHCP issues which might explain why a full factory reset got me working again for a few hours.
The alarm system has an IP address which is NOT the reservation that's been entered in the DHCP server. I've double checked the MAC address and it's not shown in the list of DHCP leases on the System Log - DHCP Leases tab either as the current IP or the reserved IP. Furthermore, hitting the refresh button seems very slow to refresh, and my Ring devices seem to be renewing their leases every few minutes.
I might be seeing problems ahead of others as I use a 2 hour lease duration when in upgrade cycles so that devices refresh their network config frequently when I'm rebooting the router. I use YazDHCP although not sure if this will be the issue.
I was having all kinds of weird wifi connectivity issues with my brand new AX86U that I never had with the AC86U. Pretty much the same as what you are describing. I ended up disabling 802.11ax / Wi-Fi 6 mode and Target Wake Time on the 2.4 Ghz band and I think that has fixed it. I didn't even know that AX mode was applicable to 2.4 Ghz? Not sure if this will help you.