How to Use Cellular Data to Watch TV and Movies on an iPhone

Open your iPhone's settings. This is the grey gear icon on your Home screen.

Scroll down and tap TV. This option will be about halfway down the Settings menu.

Slide the Use Cellular Data for Playback switch to on position. The switch will turn green. You will now use your mobile data plan when you're watching videos in your iPhone's TV app. Your iPhone will use mobile data only if you're not connected to internet via WiFi.