Need to either ensure UPnP is enabled or open the following ports.
3074
88
53
My setup has the router identify the Xbox by mac address, then puts it on a fixed IP - that way the xbox stays on DHCP. Then use the firewall rules to open those ports to the IP address you gave it and that'll be your NAT thing sorted.
If you have the crappy Sky S@GEM router, i can give you the exact things to do as thats the crappy thing I have, but sounds like you know what you're doing to work it out from that info