How Should I Extend My Network?

I currently have atnt Fiber 500 internet with the BGW320-500 router/modem. I have attached a Google mesh system to it and have 1 point downstairs and 1 point upstairs. I shut off the atnt modem and bridged(I think) the google wifi with a pass-through. I still get (double nat/strick nat) if anyone knows a fix. We just build another room over our garage about 30-40 feet away from the main house. I bought another google extender and tried installing it in the room over the garage and the google app said it was too far away from the network. How would you resolve this? I looked into the atnt extenders but was not interested with the not-so-great reviews and the fact that it needs to be 15 ft away from the mode/router. Thank you so much!