The same way you upload videos over FTP you can upload subtitles over FTP. Subtitles uploaded over FTP are connected to a media by filename. The filename of your uploaded subtitle should be "video title-language code.extension".
For example, if you have a subtitles file containing English subtitles and want to add it to the video titled "4K into the cave of wonders" you need to name the subtitles file "4K into the cave of wonders-en.srt"
The language of your subtitles, what you want displayed in the subtitle menu of the player. Use two letter codes for languages in order to display the full name of a language, for example "sv" for Swedish (Svenska) or "en" for English.
Make sure your subtitles are in a supported format, in the example above the file extension ".srt" is used. More information about other supported subtitle formats can be found here.