The Technicolor C2100T modem provided to customers by CenturyLink has a bug. By default, it's remote management interface, when enabled, will bind to port 443.

However, even when the remote management interface is disabled, port 443 will still be "reserved" and will not be available for port-forwarding.

To solve this issue, you have to temporarily enable the remote management interface on a different port, then disable it. After that, you should be able to forward port 443.