If your server is on a LAN, it will have an internal IP address. This will start with 192....etc etc. This address can only be seen by computers in the same local area network. However, your server will also connect to the internet whether it by by a router that forwards traffic to it, or a modem that connects it to the internet... this device will have a public IP address.
How do you connect to your ISPconfig server through a browser? If you put in an IP address eg 18.104.22.168, that is your public ip address ( assuming your not connecting through your lan).
If you use a domain name to connect to it like yourdomain.com:8080, try putting the domain name into this site:
It should return your public ip address.
Allowing zone transfers allows another name server to 'get' the zone records. Like a backup name server. <<--correct me if i'm wrong.