What kind of home network setup do you have? Since you mentioned only the Xfinity app was working for you it doesn't sound like your TV is a part of your home network. Most app/services like Netflix, Hulu, Youtube, etc. that you'll use on your phone also require your TV to be connected to the same local network. The Xfinity app on your phone isn't in this case working directly with your TV, it's that both the phone app and your TV are indirectly linked via your Comcast account -- your phone by either WiFi or cellular connectivity, your TV by it's a cord to your Comcast cable modem.
If your TV is new enough it might have it's own wireless chip so you can just add it your home network. Or if there's an Ethernet port connect it to your Comcast modem/router with any standard patch cable. You can also buy something like a Roku or a Chromecast that just plug into a HDMI port on your TV and once you add it to your home network via WiFi those apps you were trying out on your phone should see your TV.