You only need to forward one port to access HA via DuckDNS. In my case I use http locally and https remotely. In HA Settings - Network these are my URL’s:
Local Network: http://192.168.0.145:8123
In the IOS Settings - Companion App - Home I use the following:
Internal URL: http://192.168.0.145:8123
External URL: https://mydomain.duckdns.org:306424
In my router I have a single port forward rule:
Forward IP: 192.168.0.145
Forward Port: 306424
You can use any port number (including the standard 8123) as long as it’s not being used by other processes.
If you ping https://mydomain.duckdns.org it should reply with your current IP address provided by your ISP. You can test access with a cell phone from home by turning off WiFi and entering https://mydomain.duckdns.org:306424 into a browser on the phone.