Outlook Web Access Through Firewall

I have OWA running on an Windows 2003 server, with a Draytek Vigor firewall. I can access the OWA within the firewall but not from outside. I have seen plenty of messages saying just open ports 80 and 443 to allow access, but the Draytek has a web based admin so I figure if I open port 80 to my server I will not be able to access the f/w remotely.
So, I’ve changed the HTTP port on the IIS to 8181 (don’t know if this is an ok port or not so please advise if not) and I can still access internally by specifying the port. However, if I open ports 8181 and 443 on the firewall I still can’t get OWA externally.
Any suggestions or advice?

Thanks, Steve.