did you not say that your ISP allows you to have multiple computers online with public IP Addresses a while back? if that is the case then try disabling the NAT (or, better still, disable everything in it - especially DHCP, then plug the modem in to the hub instead of the WAN port so all computers can access the modem directly) it will save you a lot of hassle
of course if there is some need for NAT then check if it has any "firewall" options enabled that may be filtering the UDP packets, of course for any more info we'd need to know the version number of it (with any luck you got a v4 and not a 5v, because then you can do whatever you want to the thing to customise it)