If youre on a PC, open up the command prompt and type in ipconfig. Look for IP Address (or IPv4), Subnet Mask, and Gateway. Add 10 to the last number for a unique address to use on your xbox. Lets say your computers address is 192.168.1.1, use 192.168.1.11. Then try setting your console to manual again. If you forget how, System Settings > Network Settings > WiredNetwork or the name of your Wireless Network > Configure > Basic > IP Settings > Manual. Put the unique IP you made in IP Address and the computers subnet and default gateway. Then give it a shot.
"Oh bother," said Pooh as he chambered his last round.