How do i install the KPN equipment?

It looks like there is something missing. The image does not show a wall socket but an adaptorplate on which a wallsocket can be attached. I dont recognize the plate but maybe a “Monteur” on this forum does.

On your image i dont see a direct way to check whats behind this plate to be able to advise you if there is an easy way to fix it. Because there are no visible screws to loosen the plate there is not an obvious way to check behind it. It could be that its held up to the wall by the sealant so carefully prying could get the plate loose to check if the correct (if there are any) wires are behind it.

Best thing would be to call KPN and explain that you cant connect, and they can send an engineer (it might not be free, so check that aswell when you call)