Basically you’ll have to look for a version of those subtitles that match your movie / show – those can be different because releases in different countries could e.g. show different length opening/closing titles
That being said… Plex is currently preparing a new feature that’ll allow for adjusting the offset of your external subtitles (currently available with the latest version of Plex Media Server and some of the beta players). Basically this will allow you to define the gap between where the subtitle is showing vs. where it’s supposed to show and Plex will then e.g. postpone the subtitles to make up for that gap.
Here’s one particular post where this is explained in the context of Plex for Android (version 7.10.0).
Which also means it’s actually part of an official release and should also start showing up in the other clients over time.
