Hi, I've been suffering with this issue for a while not until I started reading this thread was I able to find a work around. I now have an open Nat, I'm able to join party's again and my Nat in Call of duty is open.
First look at your connection details in your advanced settings under network connections on your xbox, take note of your iP address for ipv4 and your wired mac address. Then log into your router using a laptop, pc or by downloading a Web browser for your xbox. Don't use your phone your router will need to disconnect during the process and your phone cannot handle it.
Once signed in you need to go to advanced settings and click on mac binds. Here you need to set up a static address using the ip address and mac address you took note of before its fairly straightforward once you have the info. Then find your WiFi settings found on the home page, and then click on apn. Here you need to change it from auto to manual, then add a new profile. Select ipv4 from the drop down menu then add a profile name something like xboxnet or int, input '3internet' into the APN field and click apply, you'll be prompted that it needs to disconnect to apply the changes(this is where you don't want to be on your phone doing this) after a min or two it should be successful. Set to default. Make sure you restart your router and xbox once everything is complete and with any luck you'll have an open Nat now
your mtu will still not be right, try not to let it bother you it has no effect on your gaming experience, sucks but no biggy.
