@che The config external port 443 internal port 8123 is correct. However I think you have not setup your homeassitant with secure connections Does this work from your internal network?
https://homeassistant.local:8123
If not check the letsencrypt addon for duckdns: addons/duckdns at master · home-assistant/addons · GitHub
The router is not able to convert https traffic into http. If you can connect internally with https the forwarding should work. You will get a certificate error from the internal network since the certificate belongs to the duckdns domain.