Hi Ravindra,
Login the AP and ensure that the AP has internet. (which should be getting from the at&t device)
You can verify it under Dashboard >> Internet >> You should see that there should be an IP under DNS and the "Internet" should show as "Connected".
If the Internet is not connected, then ensure the AP should be getting the IP address from at&t (which I believe is the gateway for your network and serving as DHCP server).