I struggled with this issue for WEEKS trying to use videos I recorded with my iPhone.
Here's what finally worked:
1) Convert the iPhone .MOV video to an .mp4 Full HD format. (I have Movavi Premium for video conversions, there may be other free use tools out there?)
2) Upload the .mp4 to a folder called "video" directly under the public_html directory on the server using the cPanel upload.
3) Open the product or category that you want to add the video to and select the "Source" button.
4) Add the .mp4 video using the <video></video> HTML tag. Mine look like this (for example):
<video autobuffer="" controls="controls" height="270" poster="video/leatherposter.jpg" preload="auto" tabindex="0" width="350"><source src="video/leatherclasps.mp4" type="video/mp4" /></video>
5) Save the product / category while still in Source mode
6) Clear the cache.
