The "real life" scenario where ispconfig is made for is described on page 273 of the manual "5.12 How Do I Create A DNS Zone". This expects that the server is connected to the internet directly or in other words, it is for a server or virtual server in a datacenter which has a external IP.
Your scenario is a bit different as the normal usecase though, so I will try to explain you when to use which IP addresses in DNS servers:
If you use the internal or external IP address in DNS when you run a server behind a router depends on the usecase of your network. if the dns server shall only be used to resolve "internal" domain names like mydomain.internal to private IP addresses instead of a .com or .net domain, then you use the internal IP in DNS records. But if the dns server shall be the authoritive nameserver for a external domain that is officially registered, then you have to use the external IP address.