Since it seems that we've eliminated the AC adapters from the equation, the next step would be to go into your router configuration.
I would start by turning off 2.4 GHz, see what happens, turn it back on and then turn off 5 GHz, again to see what happens. Since it started when you upgraded to Wifi 6, it's probably in the 5GHz range.
After you eliminate which band, if it's 2.4 GHz, you could probably leave it off, unless you have any legacy devices that still need to connect to it.
For 5 GHz, I would start by changing the channels to see if that clears up the problem.
All of this is assuming that you have checked all of your connections, cables, etc. If you used the cables that came with the router, you may want to swap them with the old ones. I've had crappy/defective cables come with new routers.
It's a tedious process, but will be worth it in the end.