The subtitles must conform to the rules laid out in here:
Make sure to include the language of the subtitle.
Your media files must have the language of thier audio track(s) tagges properly. i.e. the language of the audio must not appear as “Unknown” in Plex.
Then you go edit your Plex account here: https://app.plex.tv/desktop/#!/settings/account
Edit the “Audio & Subtitle Settings”
Fill the checkbox “Automatically select audio and subtitle tracks”
If you are an English speaker, set both Preferred Audio and Subtitle language to English. (Audio and Subtitle language must use the same language!)
Set “Auto-select subtitle mode” to “Shown with foreign audio” if you only want to see “forced” subtitles.
Set it to “Always…” if you always need the regular/full subtitles.
The Roku platform introduces another wrinkle: Roku have their own subtitle settings and Plex must conform to them. So make sure that subtitles are enabled globally in the settings of your Roku device.
