Regarding running Plex Media Server on a Synology NAS:
Read these two documents. They will save you a lot of time and trouble.
- Installation & Setup of Plex Media Server on Synology
- Synology FAQ - Questions, Answers, and a few How-To’s
Also, do not install the version of Plex Media Server available in Synology Package Manager. It is outdated, and, if running DSM 7, not fully compatible. Unfortunately, Synology will not update their systems.
Instead, download the current version from plex.tv (https://www.plex.tv/media-server-downloads/#plex-media-server) and perform a manual installation. The documents linked above have the necessary details.
Suggestion: Install the public version, 1.29.2.x. Based on a number of posts in the forum, it appears to be more stable than the current 1.30.x beta (which is not entirely unexpected, since it is beta…).
For the DS218play you want the ARMv8 download (Reference).
DSM 7
DSM 6