AT&T Branded Airties Mesh Extender

I bought one of these more than one year ago because we were experiencing a poor Wi-Fi signal in the areas of our house farthest from our router.  A technician sold it to us and said all we needed to do was plug it in and it would work.  That has not been the case.  I recently realized that it has to be paired to the router (we have a PACE 5268) and although I was able to get the router to see the extender, I cannot get the extender to go online.  AT&T's Smart Home app shows it's there, but "not connected."  How do I get it to work, or should I get another one?