Zyxel: DHCP Reservation

Yes, the Zyxel is a gateway (modem+router), and you have problematic double NAT as a result. If you have access to the right settings, you can put the Zyxel in bridge mode as explained on page 22 of the manual here:

ftp.zyxel.com/.../P-660RU-T1%20v3s_1.01.pdf

This is the best solution, but your ISP may block those settings. If it does, and if it is unwilling to set it to bridge mode for you, another option is to disable DHCP on the Belkin and connect the Zyxel to a LAN port on the Belkin rather than WAN. Reboot
Zyxel, Belkin, Xbox, and Xbox should get an IP from the Zyxel via DHCP (reserve this IP if the Zyxel supports that). You will then need to do all necessary port forwarding to the Xbox using the Zyxel unless the Zyxel properly implements UPnP (in which case
enable that).

In that case I would turn off UPnP and setup portforwarding exactly like this
portforward.com/.../Xbox_Live_360.htm

Once setup correctly your NAT status should never change as long as you use a reserved ip address via DHCP reservation or a static ip address

Okay cool, so to recap, I should do this dhcp reservation step first before I go into the port forwarding step. And DO I have to hard wire my computer via Ethernet to the e4200 for this dhcp reservation while having my Xbox turned on and connected to the
e4200?

Thanks

got it working turned out that i had pnp enabled on the zyxel which was overwriting the port fowarding settings i had setup. couldnt get bridge mode to work between the belkin and the zyxel but the turning of dhcp done the trick. Thanks GZA

An open NAT is no guarantee that this behavior will be consistently open for each endpoint on your network.

Can you see if your router support DHCP reservation, or IP reservation. If so, setup both console's to have their IP address reserved in the DHCP of your router. Before you setup this, make sure you reserv a unique IP outside of the DHCP's dynamic IP pool
range.

Is there any chance you can try with a wired solution?