I installed Jellyfin docker on my Synology DS920+. It's free (open source) and has apps for most devices (Roku, Amazon Fire) and phones. It is well maintained. I followed a YouTube video and it was the first time I had any experience using Docker, it worked the first time and looks fantastic. Just drop the media files in a directory on your Synology. That's what I'd recommend as a no-cost option over Plex.