@elconquistador 

If you have the wireless network name the same on the extender and the AT&T device that is why. Having the same name is convienant but the drawback is what you are experiencing. 

The extender nor the router controls what the client device connects to like a mesh network does. What you are left with is the client device swapping networks which causes network drops.

Best course of action is to have both network names different so when you are physically near a device you know what network to connect to.

@Tony Great answer. I did name them both the same because I thought that's right way to do it. I'll make the change and see what happens.

Thanks

@elconquistador 

Sounds good.