You can use the web int, SSH, or OSD...Under the Advanced Settings, H323 Settings, there is a section on NAT. If you plan to use the endpoint for internal calls as-well as external, set NAT Mode to 'Auto', If just for external use set to 'On'...then set NAT Address as the public IP you will use.
You will need to open the following ports on the firewall/router and do a one to one NAT or port forwarding so any comms from the internet targeting the public address is forwarded to the private address of the C40.
* Port 1720
* Port 5555-5574
* Port 2326-2485
If you get issues with media, e.g. one way audio/video, try disabling any H323 awareness on clever firewalls and ensure ports are statically open and forwarded.