AC1900/R7000 connect to Google WiFi

I am trying to connect to a Google WiFi 3 mesh system (hooked up to the ATT WifFi) to extend to a barn that is 350 feet from the Google WiFi in a window closest to the barn.  Currently runin a three mesh system.  I can not figure out how to connect or if it will even work.  While not new to computing and routers I am struggling with the set up.  My computer is an Apple iMac or Macbook Pro.  I want to run a WiFi camera for security purposes.  I do not have power to the barn so will use a Solar WiFi camera. The Google support guy says they do not have a WiFi extender.