Subtitles Not Showing on Screen

When I try to watch a movies from an MP4 file in which I have embedded subtitles, I cannot get the movie to show the subtitles. On the opening screen for the movie, the subtitles say “None (1 available)”. Then I go to another menu showing the streams, and I change the subtitle stream to English. After I make the change, and try to watch the movie, after trying to find the movie for awhile, a message shows on the screen that Plex can’t find the movie. Then, if I change the subtitle stream back to “None”, then I can watch the movie, but there are no subtitles.

How do I access the subtitles that are embedded in the MP4 file?

I have the same problem and would like to add some facts:
The mp4 file has Swedish subtitles embedded and is created from a DVD by Freemake Video Converter.
The mp4 file shows the embedded subtitles when I play the file with Windows media player.
In Plex I can only select ‘None’ which shows no subtitles and ‘Unknown (MICRODVD)’ which shows very bad synchronized polish subtitles that even has no correct polish characters.
I have no subtitle files in the folder where the mp4 file is located. The mp4 file is the only file in that folder.
I have the Plex server on a Windows 10 computer and all my clients also run Windows 10, including my phone. All clients shows polish subtitles.

Is there a solution to this problem?