Problems with nginx + router(Fritzbox) portforwarding

(Sorry for bad English, I'm German)
Hello,
I'm not sure if this forum is the right one to ask, but I struggle to setup a webserver with nginx:
I installed nginx (and a few other programs) on my raspberry pi and configured a portforwarding on my router (Fritzbox) (portForwarding.png). After that I used ddnss.de (a dynDNS service) to bind my IP 78.**.**.** to ****.ddnss.de.
If I type http://192.168.178.66 (local ip of the raspberry pi) into the webbrowser I see the "Welcome to nginx! If you see this page, the nginx web server is successfully installed and working. Further configuration is required." page - everything works. But if I type http://78.**.**.** into the webbrowser or ****.ddnss.de the browser tells me that it can't reach the site.

Does anyone know how to fix this problem /has some links for explanation?

System Information:

PC used for nginx webserver: Raspberry Pi 3
Operating System: Linux (Ubuntu)
http://myip.is -> 78.**.**.**
ifconfig on the raspberry pi -> 192.168.178.66
Intention: Setting up nextcloud and nginx on the Raspberry Pi 3

$ host -t A ****.ddnss.de
****.ddnss.de has address 78.**.**.**

$ host -t AAAA ****.ddnss.de
****.ddnss.de has no AAAA record

$ ping ****.ddnss.de
PING ****.ddnss.de (78.**.**.**) 56(84) bytes of data.
-> no respnse

$ ping 78.**.**.**
PING 78.**.**.** (78.**.**.**) 56(84) bytes of data.
-> no response