I'm not sure what you want to do. If you have the WiFi Network settings set to DHCP the router will assign the touch an IP address. If you want to set the router to a static IP address set the touch to static in it's WiFi settings.
Can the touch see the network if you type it's name in to the "Other Network" in the touch's WiFi Networks settings?
Just a note here, hiding the SSID is not a very good way to secure a WiFi network. It is still pretty easy to "see" the network and connect to it. It is recommended to use WPA to protect a wireless network. Hiding the SSID may be part of your problem.