1. Check what your NAS system specs are
2. Download appropriate Plex SPK file
As listed above, my DS720+ Synology NAS has an INTEL Celeron J4125 CPU which supports 64bit, so I choose the correspond download (Intel 64-bit):
https://www.plex.tv/media-server-downloads/
3. Manual Install via Synology Package Manager
4. Launch Plex
5. Enable remote access in plex
