In the end it looks like this (IP connection without splitter):
TAE box -> Adapter for ISDN (PIN 4/5) -> patch cable (or ISDN) -> Wentronic Adapter ISDN socket (left) -> Patch Dose the riser -> CAT installation cable (4-pair / 8-wire) -> Patch Dose the riser -> Wentronic Adapter ISDN socket (left) -> patch cable (or ISDN) -> FritzBox DSL entrance.
Back the network:
FritzBox network port -> Patch cable -> Wentronic Adapter network jack (right) -> Patch Dose the riser -> CAT Verlegekabel -> Patch socket the riser -> Wentronic Adapter network jack (right) -> Patch cable -> computer in the basement or Switch
I am very satisfied with the solution. A loss due to the attenuation or else what I have not.