Jogi,
Port forward on the router assigning the DHCP. There is a dependency for this to work. You will need to set up a Dynamic DNS or DDNS client in the router. Most routers have one built in. If you can reach the router over the internet, then you can have the router forward to the OBi. You will need to change the default port of the OBi from port 80 to perhaps 8080 since your router probably also uses port 80 (or change the http port number of the router).