Can't drag some video files onto iphone 6s
I have downloaded a file from Youtube (before anyone gets upset about pirating movies, it's a boring speech from a boring conference). It plays fine in itunes on my Macs, but I can't drag it into my iphone 6s. The only way I can play it on my iphone is to use VLC. This is not ideal as VLC for iphone doesn't seem to have the same playback functionality of the iphone TV app.
To be clear: the video file works fine on my macs using itunes, but can't be dragged into the iphone so that it can be played using the iphone TV app.
The codec is the same as with many other videos that work OK on my iphone - H264. However, I have noticed that the downloaded file is in M4v format, not MP4. This is strange because it's not a download of an iTunes movie file purchased from the itunes store, but a video of a conference freely available on youtube and put there by the conference organisers.
I have tried using itunes>file>convert, but no success.
So I'm stumped - why can I play this file in itunes on the mac, but can only play it on the iphone using VLC?
MacBook Pro with Retina display, macOS High Sierra (10.13.6)