First time setup

I recently purchased an airport+time capsule. We have ATT uverse. I want to set up my time capsule as a secondary router at the other end of the house from to uverse router so I can get a better wifi signal at that end of the house( the distance is about 40 feet). However I have an ATT cable box in that room. I connected the time capsule via ethernet to the cable box, and it will work as a wifi station for about an hour and then loses its connection to the internet. I am really new at networking so what would be my best option to set up?