iMac 5K late 2014 and Nvidia 1080 TI integrated Screen is black — any suggestions?

[FreddieOne]

Hallo egpu users,

after a few attempt the graphic card is recognized by the system
and is working well.
The only remaining problem: the integrated screen stays black.
Please see the following system information:

Modellname: iMac
Modell-Identifizierung: iMac15,1
Prozessortyp: Intel Core i7
Prozessorgeschwindigkeit: 4 GHz
Anzahl der Prozessoren: 1
Gesamtanzahl der Kerne: 4
L2-Cache (pro Kern): 256 KB
L3-Cache: 8 MB
Speicher: 32 GB
Boot-ROM-Version: 230.0.0.0.0
SMC-Version (System): 2.22f16
Seriennummer (System): DCPR60UTGDR6
Hardware-UUID: 9FF6BBA4-FEDF-59B4-B5C9-4E8BB8BD30E0

AMD Radeon R9 M290X:
AMD Radeon R9 M290X
GPU
PCle
x16
2048 MB
AMD (Ox1002)
Ox6810
OxOOOO
113-C408A0-730
113-C408A0-017
01.00.730
Unterstützt, Funktionsset macOS GPUFamily1 v3

NVIDIA GeForce GTX 1080 Ti
Externe GPU
PCle
x4
11263 MB
NVIDIA (Ox10de)
Ox1b06
Ox00a1
VBIOS 86.02.39.00.01
Unterstützt, Funktionsset macOS GPUFalmily1 v3

CUDA Driver 418.163
NVIDIA Web Driver 387.10.10.10.40.105
SU_TITLE

Any suggestions what I could do to get the integrated screen working
with the internal graphic card?

Really keenly looking forward hearing from you,
Freddie