The Technicolor C2100T modem (and other modems 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.

I escalated as high as I possibly could through CenturyLink's customer support system, and no one knew anything about this issue or how to fix it. I had to figure it out myself. So I'm doing their work for them and publishing a how-to guide documenting this as they have refused/failed to do so.