Synology DSM version 6 or 7?
Plex Media Server version? Listed in Authorized Devices: https://app.plex.tv/desktop#!/settings/devices/all
Make & Model of TV?
Plex client version? Also listed in Authorized Devices.
Play the video, without subtitles, on the TV and monitor playback using Plex Dashboard → Now Playing. Is the video and/or audio transcoding? Toggle the button on the right for an expanded view if necessary.
Attach a screenshot of the dashboard if possible. Drag the file into the compose window. It will attach at the cursor location. Alternately, use the Upload icon, the one with the arrow pointing up.
Your NAS may struggle with transcoding video, as you do not have a Plex Pass and therefore cannot use the GPU to assist with video transcoding.
FYI, HDR to SDR tonemapping is a Plex Pass feature, so you cannot enable it. If you are transcoding HDR video, the colors will most likely appear washed out due to the lack of tonemapping.
Additional Info:
Make sure you follow Plex recommendations for movie and TV show organization and naming. It will help when matching media to online databases and downloading cover art, show info, etc.
Plex is especially picky when it comes to TV shows. Stray from the recommendations at your own peril. You might get lucky, but at some point things will break.
Plex Documentation → Your Media
Recommend you read the following two support documents. They will help you sidestep common mistakes when running Plex on Synology NAS:
Installation & Setup of Plex Media Server on Synology
Synology FAQ (DSM 5 & 6) - Questions, Answers, and a few How-To’s
