Problems with Mac Air and ATT fiber network

Intermittently my Mac book Air won't connect to my ATT fiber network. I've been to the genius bar twice and on the phone with Apple support for 3 hours. Yesterday I bought a new Mac Air, my old one was 6 years old and getting cranky so we figured the network card was going out, abut I'm having the same problem on the new one.

Sometimes it connects and then without warning it will drop. When I try to reconnect I receive "The WI-FI network xxxxxx could not be joined." at the same time my Iphone, Ipad, and HP wireless laptop work fine. In the network list the network shows having full power but when I click on it the Network Name box is still greyed out with No network selected. ATT installed it with an Arris BGW210-700.

I sure hope someone can help me.

Susanne