Vobsub subtitles are actually pictures, while SRT subtitles are text.
Vobsub only occurs on DVD.
Your tv is not a DVD player, so it doesn’t have support to render those vobsub subtitles onto the picture.
If your Plex server has sufficient CPU power, it can help out your TV and take those vobsub subtitles and “burn” them into the video picture.
However, this means “video transcoding” needs to happen.
I’m not sure if transcoding is disabled on your server or if the server device is simply not capable of it.
Your best way forward would be to find a SRT version of the subtitles and add them to the folder with your video.
You can also convert the vobsub into SRT yourself, if you have the time: Introduction: convert image-based subtitles to SRT with Subtitle Edit
