How do I play iPhone 17 Pro videos on Photos app on Mac with macOS Mojave 10.14.5?

Yes, Mojave Photos does not import some new iPhone movie formats without conversion. I did a quick test with some recent iPhone 16 Pro movies (HEVC, HLG, iOS 18.5 and 26.1) with Mac mini 2018 Intel:

When booted to Sequoia, Photos has no trouble importing or playing such movies.

But when booted to Mojave, Photos refuses to import such movies. But Mojave QuickTime Player does open them after a few second wait, and offers to save them as new "converted" movies which Photos then can import (HLG/HDR movie thumbnails only have some cosmetic artifacts).

At quick glance with Invisor, the video specs do not differ much but the audio specs have more difference compared to the original.

So use either QuickTime Player or those 3rd party tools Richard suggested to convert the movies. But consider archiving the originals so you can use them later in newer macOS.

I'm not sure what Mojave can do, but video codecs seem hardly standardized, they change rapidly, and .mov doesn't always mean the same thing. If I were you, I'd try a different app. VLC is free and so is HandBrake. I use whichever one works at the time. I think both are compatible with Mojave, but you should check if you need an earlier version.