Uploading mp4 from mac desktop to photos so I can post from phone to Instagram

What part doesn't work? Importing into the photos app on your mac, or transferring from your mac photos app to your phone?

An alternative is to put the file into an iCloud drive folder, and sync it to your phone that way.

So we don't spin our wheels what methods have you tried and didn't work?

Also why don't you get one of the available Instagram apps for the Mac and upload directly from your Mac? Check the App Store.