They are NATing the IPv4s, i.e. multiple customers are sharing the same IPv4, which can cause problems. I called them and told them that I absolutely need to do port forwarding and asked nicely if I could get an exclusive IPv4. The support said „Of course, let me check the box in the UI for your account, should be active by tomorrow“ and done. You have to get lucky to get someone who understands your problem and is willing to fix it, though.