If you google it, you'll find lots of similar questions for O2. I think you have to contact their customer support and get that activated once.
And have a look at your IPv4 and IPv6 addresses. Sometimes you can do it via IPv6 already, just not over IPv4 because there is some translation in the way. (In case they want too much money to give you a real IPv4 address.)
Maybe you can try if you can open your FritzBox UI from the outside with your my.fritz address. I think that has IPv6 and a port forward in place (if activated).
And btw: It's perfectly fine to do it. People need storage and online collaboration. Access to their data while away.