Clearer.
When you installed Zentyal, you have defined the "admin" account (not the "root" account).
This account is the one authorized to connect to admin interface.
This admin interface can be reached either locally (on Zentyal server itself) or from internal network (your LAN) but HTTP server might be listening to specific port that is also defined in Zentyal configuration.
e.g. lets suppose you have defined Zentyal admin listening on port 1443, thne you can access it, from your LAN, typing
https://[your-Zentyal-server-name-or-IP]:1443/
notice this is HTTPS