Not sure
@Sly870 manage to resolve the issue.
I have similar problem. I have Plex Pass and my PMS running on
DS920+ buffering a lot. I tried PMS both as DS package and docker (linuxserverio and plex images). My client is NVIDIA shield and all devices are connected on 1GB/s wired connection. When I tried to access PMS from internet (mobile), I am able browse my contents in PMS but it is not play anything (stuck with loading). I checked PMS and Synology resources and looked normal.
I have another brand NAS and it plays the video smoothly on local network and remote (mobile).
I cant figure out whats causing the buffering issue with Synology