I’ve checked the file. It’s a bog standard h264 mkv and the video bitrate is only 10Mbps. There’s nothing that the fire stick shouldn’t be able to handle on its own. I’ll try playing it on the fire stick again and see if it’s trying to do something weird, but I can’t see it being that.
I use Radarr to process my videos, and I’m sure I’ve got a plugin or script somewhere to download subtitles. I’d forgotten all about it since I’ve been using Plex, so I’ll have to try to remember what it is.