Currently there are very few phones which supports 4K resolution videos. In fact, currently none of the iOS devices can display 4K resolution on screen. Neither the newest iPhone 7 nor iPhone 7 Plus supports 4K videos, iPhone SE, 6S, 6S Plus, 7, 7 Plus & iPad Pro supports only 4K recording which means you can only record 4K videos but cannot play it on device.
The cruel fact that the iPhone 7 Plus also only supports screen resolution up to 1920 x 1080 however 4K video requires to be played on at least 3840×2160 (16:9) or 4096×2160 (17:9) resolution screen.
Hence we would prefer if you lower the resolution of your video to 1920 x 1080 to get it played on iOS devices.
Currently our development team is working on a feature that automatically generate valid resolution video as per iOS device. iOS build will work automatically in the future regardless of the video resolution.