Since, restarting iPhone has not fixed your problem, you may go with video conversion process.
There are many free video convertor app available for free that you can try and convert your unplayable video file format to your iPhone supported video file format.
You may also try some other third-party media player app such as VLC For iPhone is free on iTunes. May be this app can help you to play those unsupported video file format on your iPhone 8.
If that also fails, then you can use another powerful third-party MOV File Repair tool. I mentioned MOV file repair tool, because iPhone record video in MOV format and it is a bet video format supported by iPhone.
I hope all the above-mentioned solution might definitely help you to solve ‘Unsupported Video Format’ error on iPhone 8.