Edit: There are many reports of problems with the current versions of Andoid/AndroidTV apps and old versions of Plex Media Server (1.18.x, 1.19.x). Updating to a current PMS release resolves the problems for many people.
Update to the current version of Plex Media Server, 1.23.4.4805. Version 1.18.5 was released in January 2020.
You will need to download the current version from plex.tv/download and perform a manual update via the Synology Package Center.
See the Downloading from Plex and performing “Manual Install” method section of Installation & Setup of Plex Media Server on Synology for details on performing a manual installation.
If you are new to running Plex on Synology NAS, suggest you give the entire Installation & Setup of Plex Media Server on Synology document a read. It will help you avoid many common pitfalls.
Synology FAQ (DSM 5 & 6) - Questions, Answers, and a few How-To’s also has much helpful information.
