How long you have the Problem?
Maybe your Router isn’t set to LAN 1GB?
Maybe the cables are causing a problem, unplug the cable, clean it and plug it back in.
Sometimes the system set the cable from 1Gbit to 100mbit.
Do you use Cat6 cable or maybe the old Cat5E?
Actual PMS Server have problem,
Also there are two version for Synology, DSM7.0 and DSM 7.2.2 with some problems.
Check other Treads:
https://forums.plex.tv/tag/server-synology
