okay heres how i do a little complicated but you only need to do this when you switch from ftp to xbl
Get unleash X dashboard
Once you get this dashboard, go into system then settings, then go down to network settings.
From there, switch your IP type, from static, to DHCP. Then scroll down to the bottom of the page and hit save.
Reboot your box then go back into the network settings, switch it to static then back to DHCP then go down and hit save.
Now turn your xbox off, and put halo 2 in the drive. Make sure your connection is good for xbox live (wireless etc.) Turn your xbox off, then back on with halo 2 in the drive.
Now when you go into the main screen of halo 2, like when you sign in with your gamertag, it should work, if not check your connection.
there you go, i hope i helped you in some way or another. but make sure to get unleash x, because in this dashboard, you can switch from static to DHCP.