Since you’re setting this up right. 
- Enable NFS v4 on the Syno
- Do your NFS mounts with
sec=sysand put in matching UID/GID in the Syno for user Plex (after you create it in their control Panel)
3 Open perms on the Syno so user Plex can see over NFS - media visibility is now home free.
Local setup should be just that.
- on the console of the New
- http://127.0.0.1
If not,
a. ip.addr.of.server:32400/web
-or-
b. ssh -l username -L 8888:127.0.0.1:32400 ip.addr.of.server
c. open http://127.0.0.1:8888/web and traverse the tunnel to hit it.