U-Verse loses signal

I had problems with our att modem (WiFi, U-Verse, home phone). Installed the new att 5286AC modem that att sent me. WiFi works fine, but can’t get U-Verse to work. After 15 seconds, get “Lost Signal” error message. I can go to another channel and it will play for 15 seconds, then same error message? On Demand works fine. I hit reset on the modem and no resolution.
Any ideas of how to fix the issue?