I have discovered something very interesting that might help explain why this is happening.
When I rang Dlink support (in Malaysia I think), they were able to confirm the problem. However, once the Dlink person in Australia called me to help diagnose the problem, he was able to connect to it.
I then tried to connect to it from work and discovered that I could connect to my webserver in my NAS using SSL on port 443 but couldn't connect to the router on SSL port 8181 as I would get a proxy problem/error.
Note that both do not have valid certificates.
I wonder if this is some sort of incompatibility with certain proxy servers and the HTTP SSL server in the router?