ISPConfig is a server controlpanel, so it manages multiple servers. Port forwarding and traffic routing is done by a router, so that not ispconfigs job.
If you want to host multiple server with just one IP address, you have to configure the port forwarding in your router.
I recommend that you install just a single server if you have just one IP. Multi server setups are made for datacenters were each server has one or more IP addresss.