Did you get that version from Synology’s App Center?
Try installing a recent version from Plex’s download page (using the option to manually install it).
- if you’re on DSM 6 → https://www.plex.tv/media-server-downloads/?cat=nas&plat=synology
- if you’re on an early version of DSM 7 → https://www.plex.tv/media-server-downloads/?cat=nas&plat=synology-dsm7
- if you’re on DSM 7.2.2 or newer → https://www.plex.tv/media-server-downloads/?cat=nas&plat=synology-dsm72
IIRC, versions between 1.42.3 and 1.43.2 had a security issue and are not able to connect by any app remotely.
https://forums.plex.tv/t/important-security-update-for-plex-media-server-v1-43-2-and-earlier/942319
