said by TheJaguar3:Any idea as to why I am getting this error message?
Yes. The Set top Boxes use a combination of TCP and Multicast to deliver the video. When you change channels the box initially connects to the TCP unicast video stream. After 10 seconds it joins the multicast stream. You are not forwarding multicast through your router so it breaks. I would not recommend you do that anyway. There is no reason to flood your router with all that video traffic. It buys you nothing.
Off-hand I can't recall the issue with U-verse Live and if it requires you to be on the same network segment with your Set Top Boxes, unless you are trying to watch DVR video or something like that. You do have to be on the same network segment to be able to remotely control your Set Top Boxes via the U-verse mobile app. It can not detect the receivers otherwise. It does some sort of broadcast or multicast to find them. That traffic will not pass through your router.