mp4 from Galaxy S21 works, but S23 mp4 does not

Ok thanks. What's actually happening is that the lightsail server I have can't play it. It plays audio, but no video. So I'm thinking that Windows Media Player on that server isn't able to figure it out. I tried running an update on that, but still no luck. The Galaxy S21 videos were fine

Given that the video is downloading correctly to your new device (because it is trying to play it) the error message is telling you that it doesn't know how to play the video.

I'd suggest searching for how to add additional video codecs to your Samsung device. You might be able to determine the codec required by downloading it to desktop computer (Mac or Windows) and looking at the file properties.

I'll look into it. Side note: The exact same video from my phone plays correctly on my laptop using Windows Media Player. Also, I installed VLC on the remote machine and it didn't play on that either. And windows explorer shows a preview image of the Galaxy S21 video, but does not show one for the S23 video. So I'll have to see if I can make a change on the phone itself