It’s possible that you already have DS-Lite. So you won’t get a public Ipv4 adresse to forward on.
You can easily check that by browsing to www.wieistmeineip.de , if you’re shown a Ipv6 adresse, you have DS-Lite.
In that case you have to call the supportline under 0800 66 88 66 to get switched back to Ipv4 only.
Don’t worry, there are people with english skills.