For you or anyone else trying this to see if it helps you out, the command for ffmpeg I would suggest is:
ffmpeg -i "input file.mp4" -c copy -movflags +faststart "output file.mp4"
The -c copy part is necessary to make sure this is just a lossless remux and not a lossy re-encode. The -movflags +faststart is not strictly necessary but is generally a good idea for .mp4 videos on the web: it makes sure the output file is stored in a way that’s simpler and faster for browsers to stream.