I have a Google nest Wi-Fi mesh system. I originally had 1 router and 2 access points. I ordered a second router by accident instead of just an access point. I researched and see it says it can be added as an access point. I remember having trouble with one of the other access points being added but finally got it added. I am currently having trouble getting the router added to the mesh system. I found the other conversation that gave the tips for connecting the 2 routers by cable. I also saw the tip for using a different device to set it up than the one I had been using. None of this works. Are there any other tips.
Make sure you have no VPN enabled/ connected; not any 3rd party product, nor the "Apple iCloud Private Relay" ( https://support.apple.com/en-us/HT212614 ).
Make sure the Google Home app has been granted permission for "local network access" ( https://support.google.com/chromecast/answer/7172427?hl=en ).
IPv6 on Nest Wifi has caused strange problems lately. One thing to try is to disable IPv6 ( https://support.google.com/googlenest/answer/6361450?hl=en ), add new Nest Wifi unit, then leave IPv6 off or enable it again.