I have a movie with 2 mp4 files but only 1 srt file. I’ve been able to get the subtitles working for part 1 in two different ways: [1] Asking Plex to find subtitles; and [2] Uploading subtitles to Plex. I’ve also tried [3] Naming the subtitle file “Name.srt” to go with “Name - part1.mp4” and “Name -part2.mp4”. That doesn’t work even for part 1.
OK. I used ffmpeg to concatenate the 2 mp4 files together. That seems to have worked. But…
The subtitles are about 15 seconds too late. Plex documentation says it’s possible to adjust the subtitle offset, but the instructions to do so say to “open the Playback Settings”. I either can’t do that (which I think is what’s happening), or possibly I am doing that but the only subtitle options it gives me is to enable/disable them and to choose (download?) other subtitles (which, amusingly, seem to be split into 2 parts. I’ve tried via Roku, Firefox, and Chrome, and only got the latter options on Roku.
Check whether what you have is actually a “movie”. Sometimes it began its life as a “mini-series” and therefore needs to go into a “TV Shows” library in Plex.
Which also removes the hassle with the 2-part file, since it can be kept in 2 separate episodes.