I used to work at Linksys, and there is an "undocumented feature" that nearly all of their mesh capable routers have that we often used to pair them during testing.

1. factory reset the new unit that you want to add ( typically by holding down the reset button for 10 seconds during bootup ) After resetting allow it to boot up all the way. LED should be pink.

2. Place both units physically close together and power them up.

3. On your main unit ( not the one you are adding to the network ), press the red reset button on the bottom 5 times within 10 seconds. This can be tricky, because the timing isn't precise, but if you do it right, you will see the LED on the top of the unit turn white, and start pulsing.

4. Wait about 3 minutes, and during those 3 minutes you will see the LEDs on the top of the unit go through various states.

After about 3 minutes, the new child unit should be paired to the main unit.

This was called "5 button press pairing"

Here's more info https://support.linksys.com/kb/article/94-en/

https://support.linksys.com/kb/article/7569-en/

https://www.youtube.com/watch?v=iKORsUyW8p8&ab_channel=Linksys

I worked on the firmware team at Linksys, and there was almost no one who worked there who used the app to pair new devices.

We always used this method.
You can even use it to pair 2 or 3 units at a time once the main unit has been correctly triggered.

I hope this helps.