Sergio
There is a differnt configuration screen, depending on which type of service you have.
For example:
In Configure this server, for server and gateway dedicated,
on the External Interface Configuration page, if you select the static IP choice, you should see provision to enter rthe static IP and gateway IP, which would generally be supplied by your ISP if you get the modem preconfigured from them, or otherwise as you have configured the modem yourself.
Hope this helps
Ray Mitchell