Hi @Nici1990, welcome to our forum!
I have translated your question to English as this is an International forum.
If the device is only set to demo mode and is not a demo unit, you can type try and type *#*#73556673#*#* on the phone pad and if everything works fine, you should end up on the home screen again and the demo mode should be deactivated.
If there is no change, go to Settings > Lock screen & security > Device admin apps > uncheck the box next to "Retail or Demo Mode option".
If you still are not able to remove the demo mode on the device, please get in touch with the retailer where you bought the device as it may be a retail demo device, and they are not meant to be sold.
Keep me posted!
