We would recommend using Migaku Player to watch local video files wherever possible.
You can follow this video guide for how to use Migaku Player: https://www.migaku.io/video-guides/the_migaku_player
You can follow this video guide to see how to upload subtitles in Migaku Player: https://www.migaku.io/video-guides/font_settings_and_subtitle_settings_in_video_content
If you are using Migaku MPV, you can add subtitles by dragging them onto the MPV window and pressing "b" to open the Migaku MPV program (if you have the Migaku MPV addon installed).
Sorry this took too long to respond I think there was some issue with the forum notifications.