We're excited to announce that we've fully migrated the Xbox Forums to their new home, here on the Microsoft Community forums!
As of today, going to forums.xbox.com will redirect you here to the Xbox area of the Community forums. We've moved all of your existing threads over so you can continue and start conversations with the new forums experience.
We sincerely appreciate the feedback you have provided over this transition and can't wait for you to check out the new Xbox Forums!
Initial diagnosis: double NAT, which prevents you from getting open NAT. Your "modem" appears to be a gateway (modem+router), which if so creates a problematic double NAT situation when combined with your router. To verify double NAT is the issue, please
log in to the web interface of your router and note the WAN/Internet IP it reports (WAN IP, not the LAN IP!). If that IP differs from the one reported by
http://www.tracemyip.org, then your Xbox is behind multiple layers of NAT (double NAT) and getting open NAT will require special steps.
Basically, you have 3 options. One, you can put the gateway in bridge mode. Google the model of your gateway plus the words "bridge mode" and you will probably find instructions--e.g.
Once you do that, if you have DSL, you will need to configure your router to perform the DSL login (via PPPoE etc.) using the login/password from your ISP. Otherwise, you will not have Internet access. If you have cable, you should be good to go. Once
routing has been disabled on the gateway, use your router to forward ports via UPnP, manually, etc.
Two, as an alternative to placing the gateway in bridge mode, which can be complex for the inexperienced, you can try putting the WAN IP of your router in the DMZ on the gateway then use your router to forward ports via UPnP, manually, etc.
Three, you can get rid of the gateway and replace it with a modem-only device instead.
The Xbox support site actually has a decent guide for NAT problems: