Apple Airport

You won't be able to do any of this if the AirPort Extreme is powered off.

Just power off the AirPort Extreme and connect your WiFi devices to the Xfinity network. That all that you need to do for now.

The whole idea here is to find out if the Xfinity WiFi is working OK. If your devices perform better when they are connected to the Xfnity WiFi, then you really don't need the AirPort Extreme.

Not sure how old, an "old" AirPort Extreme might be, but if it is 5-6+ years old, it is time for it to retire.

After reading this today I was inspired to try our Xfinity modem/router solo (powering off our Airports).

In my observations (today only) I have found that unplugging our Airport Extreme allowed the Xfinity router's network speeds to double those it provided before. So I rearranged the entire setup to no longer need the Airport Extreme.

On a separate note I then experimented with extending the Xfinity network with our 2018 Airport Express. When extending via Ethernet the Xfinity network crashed. I have read here that you must extend a network wirelessly. Now I just need to accept that there is no need, since the Xfinity's range when solo is adequate.

Apple even says in their own material that simpler is faster. Less is more.

Guess I should just sell/donate them.

When extending via Ethernet the Xfinity network crashed

Then, the AirPort(s) were not configured correctly. You cannot use the "extend a wireless network" setting on the AirPorts and then connect using Ethernet. That will create a huge feedback loop that will in effect, crash the network.

I have read here that you must extend a network wirelessly.

You might have read this, but it is not correct. If you want to use Ethernet on the AirPort(s), you will need to reset them and then set them up up again. The setup utility will suggest the correct setup, which may surprise you when you see the settings for the AirPort(s).

Now I just need to accept that there is no need, since the Xfinity's range when solo is adequate.

Then, I would leave well enough alone, unless you want to spend some time experimenting with the AirPorts connecting using Ethernet. This would really only make sense if the AirPorts are located a few rooms away from the Xfinity router and connect back to the Xfinity router using a permanent wired Ethernet connection.

Power off the AirPort Extreme and use the Xfinity wireless for WiFi and streaming for a day or two.

Report on your results.

Pull the power cord from the back of the AirPort Extreme or pull the power cord plug from the AC wall socket, whichever is easier.

Nice work. There is a lot to be said for keeping things simple.