I pass through German airports quite often.
In addition to Buenchico's advice (always very good advice), the following may help.
Usually, once you connect to the wifi service, you are not authorised to use the service until you have logged in (I'll get to that in a moment).
On a phone, it will often bring up a window or alert, as soon as you connect to the service. That alert requests that you sign in. On my Mac, that window should pop up automatically, but often does not, so I sometimes have to launch a new browser, and then it usually pops up.
If it doesn't I sometimes switch off wifi and then switch it on again, and try to log in to a particular page. That often triggers the login box. I sometimes have to disable my pop-up blockers as well.
Once you have the login thing, it asks for an email address, or sometimes your phone number I have a series of disposable email addresses I use for that purpose, as they are spam-attractors, though that should change after May of this year when the GDPR rules come in.