Rather than messing with dynamic dns I would do this to start with:
Find out the IP address that your router is connect on at the moment. You can probably find an internet web tool to find this out.
Make a note of this.
The standard ISPConfig3 setup serves web pages on port 80 (as with most web traffic) and the control panel serves up on port 8080. So go into your router and set up port forwarding so that everything on port 80 and port 8080 gets forwarded to the local ip address of your server. If it has the option to forward all ports this might be a good idea.
The table will look something like this:
80 Forward to 192.168.0.100 < replace that with the server lan ip
8080 Forward to 192.168.0.100
Then, from a browser on a computer on the internet, not in your lan, go to:
< replace the ip address with your router's one
That should log you into your control panel
<replace with your routers ip
Standard web traffic comes in on port 80 which should be forwarded to the lan ip of your ispconfig server. Your should get the page saying 'it works!'
If this is working then you're halfway there.