Hi @Mike
Firstly, I want to emphasize that you did the correc thing by trying to check that I am not behind CGNAT, since that is the most common issue!
I just checked the IP from my (Network/WAN) overview screen, and looks like I have an IPv4 address on my mob1s1a1-interface in the subnet that I mentioned earlier. However, looks like the IPv4 address is not always in the same subnet than what the whatismy .com-service provided for me earlier.
It might have something to do with the fact that my ISP is the low cost subsidiary of DNA OYJ, and thus they have access to a different block of those public IP’s 
I would like to emphasize that I can use the port-forwarded website from within my LAN& when connecting to the site via Duckdns-domain via an VPN, once I have manually updated the public IPv4 addresses correctly to the Duckdns. org-console.
I believe the issue might be with the RUTX50-routers DDNS-client, since it does not update the changed public IPv4 address to the DDNS provider (Duckdns. org). Here is what the DDNS-console on the RUTX50-looks to me:
Please note that I have added spaces to some of the domain-names, since this platform has a limit on links in a single post…
Thank you @Mike for taking the time!