Maybe you want to start with sharing some more details?
You installed the Plex Media Server on your Synology through their App/Control center?
The Plex Media Server app is running?
You’re able to access it through your browser using the Disk Station’s local IP address (http://[Synology IP address]:32400/web) to complete the installation?
Yes I have installed the Plex App via the Synology App - I’ve done both manual and auto install.
The Plex server is running but I cant connect to the web page using 32400/web
I’ve attempted it several times, Ive tryed via Wifi/Lan and mobile phone.
Do you get any error message in your browser?
During the initial setup it will only be available within your local network / while on the same subnet).