Story time: this past summer I wanted to travel from Canada to Florida (3 days 2 nights) with just me and two young kids. To save on airfair I decided to drive. I bought myself the dlink DIR-505, one of those vehicle power bars, packed my laptop running plex, plugged in my portable drive that houses all my media, and I had a mobile hotspot the kids connected to from each of their devices to watch movies using plex, it was intuitive enough for them to use. With each having headphones, I even streamed my music to the car's bluetooth using plex. We went for 12hrs on our longest day, it was a huge relief from the boredom at times.
I've never had a problem with subtitles though, can you explain what you mean?
Has always worked perfectly, even with 4k high-bitrate video. Supports subtitles and can even fetch them for you.
PS: and it has Opensubtitles support. I do sometimes have to resort to using VLC to grab subtitles (for a diff language) but even VLC sometimes does not work. Then I gotta resort to, well, a web browser to grab the sub.
[1]https://forums.plex.tv/discussion/288806/plex-losing-subtitl...