That certainly hasn't been easy for you ― let’s check this out.
What is the exact message when you’re setting up or adding another Nest WiFi router to your main Nest WiFi router? How far are they from each other?
The first thing to do is to make sure that your main Nest WiFi router is connected to a modem via the ethernet cable. Once the setup is complete, you can now add the other Nest WiFi router as a point but make sure you do it near the main one. You can transfer it to where you want it to be once done.
Follow this link Set up Google Nest Wifi to get a better feel for the setup.
Keep me posted.