I thought I also have a problem with NAT loopback not beeing supported until I found your post.
I have the HomeAssistant behind the HG659 and can see the port open on 8123, can access the HA from my phone on data using the duckdns URL from the outside of my LAN. (Naturally I can access the HA from inside the LAN using it's IP address.) Just cannot access the HA from inside my LAN usinf the duckdns URL.
What are your settings to get this working please?