Well create a backup config, try stock, if that does fix then try factory reset.
You'll still have your config to go back to.
For testing you want to be as close to out of the box as possible. So preferably nothing connected.
Try fixing your 5ghz to 36-48.
Some devices like Amazon Firesticks and LG tvs don't work on certain channels.
I had to fix mine on 36-48 to get a consistent connection. (Yes there were other people using these channels too)...
I thought there was a driver update that fixed that? Might be a bit difficult to fix on the XT8.
Question is, if you can find a B version, will it work? I've been looking for one that is a B version but difficult to confirm they are B versions.
Those setting were from my RT-N66U running very old Merlin firmware. I haven't tried in on my XT8 yet and I don't have IPv6.
Was just showing what works for me.
If there is interference on the 5ghz2 band. It gets disabled for a short while and has to revert to another band. 2.4ghz seems to be the default. I turned 2.4ghz off but it took a while and a couple of reboots for it to actually deactivate.
I got these two units again and really stable now with...