ffmpeg does work, perhaps you didn’t quite get the command right.
ffmpeg -i UnOptimizedFile.mp4 -c copy -movflags +faststart OptimizedFile.mp4
If the filenames have spaces put them inside of quotation marks:
ffmpeg -i "UnOptimized File.mp4" -c copy -movflags +faststart "Optimized File.mp4"
I would probrably copy or move the file out of your Plex library while you are doing it, just in case. When you move the file back into the library, you might have to “Analyze” the file again for Plex to get the new information.
