Photos cannot play this video because it uses an unsupported format - is there a solution?

I've discovered that any video imported to my Mac from about May 2019 onwards that wasn't recorded on my own iPhone won't play. I don't know how long this has been an issue (I will have played them on my phone and then downloaded them assuming they would be fine) and have just discovered as I'm housekeeping before migrating to new Mac. They were all probably sent to my via WhatsApp and then downloaded to the Mac. Videos from the same sender sent earlier than that are still playing. When I try to open the video I get the unsupported format message, but can't find any way to find the files in Finder to try another player. The Photos app knows the length of each video.

From the same date I've discovered that although I can open photos sent to me and then downloaded to the Mac, I cannot edit or export them - I get a message saying that the file is not present.

I'm concerned that if I try to migrate I'm going to have hundreds of photos and videos that won't migrate and any help to recover would be gratefully received. I'm currently on High Sierra on the 2010 Mac and Photos 3.0