On your setup, the xbox should get it's dhcp ip adress from your internet service provider.
Some providers only allow a limited number of adresses.
Some providers check the mac adress of the connected equipment and only provide an ip if the adress is correct.
The xbox mac adress is stored in the xbox eeprom. Maybe, due to your softmod, you have a virtual eeprom enabled with a mac adress that is seen as invalid by your internet service provider.
So, I would check the xbox mac adress. Most modern dashboards have an option to check this out.
You might try to disable the virtual eeprom and see if that makes a difference.
Connecting multiple devices to the internet is better done with a router.
Those boxes have a buildin dhcp server and usually function as firewall as well.
The router only needs 1 wan adress from the ISP.
regards.