Router management UI is available in HTTPS as well as HTTP (using a self-signed cert, you’ll get a warning the first time). We kept HTTP open for convenience to the user.
When a page is blocked, the router tries to display a “blocked” page. This works if the accessed page is in HTTP. For pages blocked over HTTPS, the router still tries to send the “blocked” page. But since the router does not have the right SSL cert for the requested web site, your browser will either deny the page or show a SSL cert warning.