Reach internal web server via DDNS domain instead of admin pannel

Hello,
unfortunately I have the following situation at the moment.

The thing that works wonderfully is:
Behind my router GL-MT2500 I have a web server running. For this I have set up two port forwarders for port 80 and 443.
In addition, I have set up a ddns service (via luci) because I use duckdns.org.
The nice thing is that I can access the web server from outside, i.e. from the WAN interface. So with xxx.duckdns.org

But if I try to do this from internal, i.e. via LAN interface, I unfortunately end up on the GL.iNet admin panel. So the website that can be reached by default via 192.168.8.1, although I use the domain xxx.duckdns.org.

Why does the admin panel also respond to xxx.duckdns.org.
So how does the nginx server know from the GLi-Net admin panel that it should also listen to xxx.duckdns.org. Or how can I prevent this?
I can’t figure it out from the file /etc/nginx/conf.d/gl.conf.

First of all you need to leave such access to the webserver on the router, to administer the router, so if your router is reachable on https, do not do this for the 443 rule (have 80 and 443 in seperate rules).

Do not do the port forward on the GL.INET interface, we;ll do it on OpenWRT - there we can enable the “Enable NAT Loopback” option in Advanced settings.

Example rul from my external box. Forwarding a port to port 80 of the raspberrypi where I have a web server running.
So normal forwarding rule

On the Advanced tab:

The above should allow you to reach the webserver internally if you use the domain name.

Simon