Port 443 was already taken by the router's Remote GUI Management. To free up that port:
To change Technicolor's port 443, first log in to the router's admin interface and navigate to Advanced Settings or WAN Services to locate the remote management settings. Then, enable the Remote GUI and change its port to a different one, like 4433, then save the changes. After this, disable Remote GUI (if desired) and save again. This action releases port 443, making it available for your port forwarding rules.