Make sure the HDMI cables are connected correctly and that your TV and AT&T modem are both switched on. Attempt restarting both your modem and your TV if the issue still exists. You might need to contact At&t customer service if you're still having issues. To check if that helps, you could also want to try a new HDMI cable. If the problem notice persists, it's possible that your TV is incompatible with the HDMI signal coming from your cable box. In this situation, an RCA cable may be required to connect your cable box to your TV.