Thank you very much for your efforts!
It seems there is no Solution for my Problem…
As I was told, the DI2 setup in my Bike requires the Shimano STEPS Protocol, which Wahoo does not support on any Elemnt as it seems.
As long as there is no Workarround to have my DI2 use the standard DI2 Protocol or to train my Roam to support STEPS I will have to live without Gear information or the ability to switch pages using the DI2 Buttons.
But maybe someone has a hint or knows a Worarround?
Or maybe Wahoo adds support for Shimano STEPS?