Can you access your wan ip when you are somewhere else than on your own lan?

If not, then this is probably just that your router does firewalling and nat is such order that you can access admin interface from local network via wan address.

If yes, then router has some serious misconfiguration.

Blog makes valid point, but why on earth there would be any current Linux distribution without usr merge?

EDIT: Especially when every major Linux distributions have already implemented usr merge long time ago.