Just to confirm- you don't have a blue network light on constantly on the 323 when you connect to the new gateway?
Regardless of any DHCP settings, this light should be on when connected to a network I would have thought. Is it something silly like you need to turn the LAN ports on in the interface for your gateway, or perhaps try a different cable? Straight vs crossover? Although I would expect the 323 to auto negotiate that.