The security certificate is only valid for your DuckDNS domain, it is not valid for the IP address and there is no way to add an exception as you can in a browser. So you cant use the IP address unless you set up a reverse proxy to allow you to use http <ip_address>.
You will have to use the DuckDNS address and rely on NAT loopback for local access or set up a reverse proxy.
Also you should have included this information in your other post.
Please keep all related information in the one post.