Apple's new TV app on iOS 10.2 struggles to play videos that have been ripped from DVDs and other sources and loaded into iTunes for Mac. It’s unclear whether this is a simple bug, but that didn't stop people from flocking to Apple’s Support Communities forum to express their disdain for the app.
The TV app has replaced the old and mostly useless Videos app.
Thankfully, dedicated apps let you easily play non-iOS-friendly media on iPhone, like FireCore's versatile Infuse video player. Handbrake for Mac, which recently exited beta after 13 years, packs in a comprehensive set of tools for easy video conversion with handy presets for turning ripped DVDs into an iPhone-compatible format.