You checked if you have a DS-Lite contract? Those are very popular. Especially with O2, Vodafone and the fiber ISPs.
The problem: You get only an IPv4 CG-NAT IP and a regular IPv6 IP. If your ISP (for example at work) did not configure an IPv6, you will not be able to connect (A and AAAA DNS records).
I assume you set up the port forwarding in your Fritzbox (under Internet > Freigaben > Portfreigaben)?
If it has the proper external port connected to your internal port it should connect.