https://www.reddit.com/r/appletv/comments/vuthg3/how_to_play_ipadiphone_files_on_the_apple_tv_with/
Above Reddit thread had the same question.
Guess you can use Airplay from your iPhone to the Apple TV but I guess this is not the most optimal situation (also quality wise).
Also someone mentioned to host a FTP server on your phone so that you can connect from the Apple TV … Guess very cumbersome because the FTP app has to have access to the files on the phone.
Infuse shines the most (at least for me) when using a NAS (wired) with SMB shares hosting full UHD movies.

Enjoy! Please try the app Infuse next to enjoy movies from your own collections