The newer Apple iPhones no longer record in h.264 format, so you will need to change the settings in your device in order to record video that's compatible with the platform.
To start, go to Settings>>Camera>>Formats and select Most Compatible before recording your video. This will ensure that your video will be converted to h.264 format and will be properly rendered in the platform to judges and administrators.
If you’ve already recorded your video in High-Efficiency format, iOS 11 will automatically convert your video to Most Compatible (h.264 format) upon exporting the file to a non-Apple device or non-Apple/iCloud storage (i.e. Dropbox, etc.).
Watch this video for guidance: https://skild.wistia.com/medias/zhnxrra5vg
How to disable High-Efficiency format: https://www.macworld.com/article/231501/how-to-disable-heif-and-hevc-formats-in-ios.html
Apple Support: https://support.apple.com/en-us/HT207022
Also, if uploading directly from your mobile device, be sure to export the video to a folder and ensure that the filename ends in .mov, .wmv, .avi or .mp4, otherwise you will get an error “Please use a different filename.”