thx for your effort.
Just found out that my ISP is using an IP range resrved for CGNAT (whatever that is...). I think this means, that these adresses are not reachable trough the internet. There has to be some mapping in between. And therefore services like duckdns will not work properly. So thats the point why Unraid or whatismyip.com shows different WAN IP as my ISP.
Maybe I can come up up with a workaround...
Thx for the help!