Setting up DuckDns with a Nest WiFi network

I’m trying to get my Home Assistant running on a Raspberry Pi 4. It’s been very straight forward until I tried to get it expose it externally.

My Pi is connected to a Nest Wifi via ethernet and my Nest Wifi is connected to a BT Smart Hub. Any advice on how to set up the port forwarding so that I can connect to Home Assistant when I’m out of the house would be very welcome.