The router in question is a Zyxel PK5000z.
No port forwarding.
Everything runs correctly.
When remote management is enabled using the Zyxel recommended default port of 443, secure remote access operates correctly.
If a machine is placed behind the router in the DMZ, then remote access to the router becomes inaccessable regardless of port or protocal.
Disable the DMZ and everything is normal again.
Why does the router not trap messages intended for port 443 instead of passing them on to the DMZ?
Do you have the port open in your firewall? If you don't have one you should, and next since you're using SSL do you have a trusted certificate for authentication purposes? This shouldn't be an issue, but could be if Zyxel requires one for this port. Obviously you don't want just anyone to have access to your router and be able to configure it remotely, talk about an easy entrance right into your network. Some routers enforce certain security restrictions such as this in order to help protect people from unknowingly opening themselves up to getting owned by some hacker.