Cannot set static IP address

Can you post the actual firmware version?  (which would be 5.3.11 for a v2, and 4.1.14 for a v1).  One reason is to confirm the platform.

Dynamic vs Static IPs Most routers assign dynamic IP addresses by default. They do this because dynamic IP address networks require no configuration. The end user can simply plug their computer or console in, and their network will work. When IP addresses are assigned dynamically, the router is the one that assigns them. Every time a computer or console reboots it asks the router for an IP address. The router then hands it an IP address that has not already been handed out to another computer or console. This is important to note. When you set your Xbox 360 to a static IP address, the router does not know that the Xbox 360 is using that IP address. So the very same IP address may be handed to another computer or console later, and that will prevent both devices from connecting to the internet. So when you asign a static IP addresses, it's important to assign an IP address that will not be handed out to other devices by the dynamic IP address server. The dynamic IP address server is generally refered to as the dhcp server.