Hi, Peter!
Is SAP server really listening on it's external IP address and not on localhost?
You can check it by telneting to it's IP and port 32<instance number> (3200 for 00). Or check netstat command on a server.
If you need a fast solution I may suppose that you'll need some extra actions for routing the connection from external server IP to internal one or making it listen on an external one. But I believe you'll prefer a more secure way of living, so how about configuring some VPN on a server?