I currently have the following problem:
I have built a network with a FritzBox 6490.
On the LAN is the RaspPi with HA and a Windows PC.
I can reach the RaspPi HA from the PC.
It works https://192.xxx.xxx.xx:8123 and also https://homeassistant.local:8123
WLAN network is an Android tablet and also a smartphone with Android.
On the tablet I have installed fullyKiosk and reach HA via the URL https://192.xxx.xxx.xxx:8123, no problem also via the browser Google Chrome I can reach HA with this URL.
With the smartphone I can also reach via the google browser via the
URL https://192.xxx.xxx.xxx:8123 to reach HA.
Smartphone and tablet are in the WLAN home network.
Now I have the following problem:
From the home network I can no longer reach HA with the Homeassistant app!
Only if I am no longer connected to the smartphone in the WLAN I can reach HA via https://xxx.duckdns.org.
I have also uninstalled and reinstalled the APP.
not sure of your problem cause but certainly seems related to http section.
I was thinking HA was providing the ssl cert for your duckdns when you connect internal but honestly I not sure if that would create issue or just give browser warning.