Ports closed. No access from outside home network. But where's the problem?!

[Schmu]

I totally agree! IPv4 and real Dual Stack (DS) and not DS-Lite should be the focus.
@kittysuzuki You should ask your Internet Service Provider for Dual Stack first and then continue.

@kittysuzuki

I have tried it with myfritz but I only ever end up on my fritzbox interface

Please make sure you change the port on which the FritzBox web interface is available. By default this is port 443. Therefore you will always end up on the FritzBox when accessing your external IPv4 on port 443 (HTTPS).
Changed that port in the network settings for for FritzBox and it might work already with MyFritz.