When I tried to download the Xfinity App on a TV on a Smart Tv in a house we were at for vacation, it would not allow us to watch anything because their wireless was not Xfinity in the house. You should be able to watch the TV app on any smart tv as long as you log into you Xfinity account. Can this please be available for people that pay for Xfinity internet/wireless?
What type of App. Was it the Roku app? If so, it is designed to replace a standard cable box or DTA, not let you watch TV on the road. The video access is tied to your home Internet Comcast service. If it doesn't detect it on your home network, it will not work.
For out-of-home viewing, you are limited to the iThing and Android apps, and typically limited to TVGO rated channels and your own recordings (if you a have X1 with cloud recording enabled.)