Can you open it in QuickTime? How about VLC? QuickTime allows you to trim. iMovie is probably the most appropriate basic tool for editing unless you already know FCP.
Opening the file in QuickTime is a good suggestion. If it can open it, it should be able to export it to another format (or a more compatible mp4).
That said, I’ve never run into a video file that HandBrake couldn’t deal with, so that’s very peculiar.
If nothing else works, you might try uploading it as a private YouTube video. If YouTube can’t convert it, it’s probably hopeless! Then you can download the converted file from YouTube using Downie or YouTube-dl.
Oh, that reminds me, Permute is another app to try. It’s part of SetApp, if you’re a subscriber.
Some types of MP4 aren’t readable in FCPX due to codec incompatibility. Decent overview in this thread. You could probably use a free converter like MPEG Streamclip to make a FCPX-friendly file you can then edit.
Sadly, Apple has not improved QuickTime “X” to the level of QuickTime 7, which could edit out if the middle of files as well. (That may have required a “Pro” license.)
Even more sadly, Mojave is the last version of macOS that QuickTime 7 will work with, since it is 32-bit.