I'm gonna make this quick, b/c I don't believe I'm going to try to answer your original question, b/c I don't understand IP and port forwarding at this time, so I'm not going to go down that road.
I have an old AMD server that I installed ISPConfig 3 on, and I left it DHCP. If you want to be seen by the outside world, I suggest you do the same. I don't run the server behind any firewall at all. I'm not recommending this to anyone, I am just telling you this, b/c I know it works and I don't want to deal with router problems. What I recommend is that you use DHCP, since you have a single server setup and everything is installed on that one server.
There is a company http://www.dnsexit.com/
, that provides a DynamicDNS client, and I use this on one of my own home servers. Their service is free, and I give them kudos for that. They are a nice company. Here is a link to tell you about their DynamicDNS service: http://www.dnsexit.com/Direct.sv?cmd=3
If you want to reinstall everything, you can do an "ifconfig" after the initial install to find the IP provided by your ISP. This is the IP to use with dnsexit.com for the initial IP. Install the client, and when your IP changes, dnsexit.com will be updated with your new IP.
If you don't want to reinstall everything, change your /etc/network/interfaces and /etc/hosts back to DHCP.
Keep in mind that for every secure site you want to run on your server, you will need a unique IP to run it on. This can be an issue if you only have one IP and don't fully understand IP forwarding.
I hope this helps... Please let me know, if I can help further...