Not sure how to remedy this

So i use smoothwall www.smoothwall.org as a router and firewall, and it has its own dhcp server which assigns an ip address to each local machine, and i am trying to run a dedicated Linux server and it will not let me bind the udp port to the external address, only the one assigned by the smoothwall dhcp server, which obviously is not public. The ports are all forwarded but it is only listening to other local machines, is there some sort of solution to this problem other than getting another switch and putting it on a completely public ip address?

Use a static internal IP, espicially when doing port forwarding.