old Thinkpad Advanced Dock with R500 issue, help needed

I purchased an old Thinkpad Advanced Dock, which I want to operate with a R500. The Dock seems to work after all, the two internal fans work very loud, but the desktop graphic cards do not display an image. I tried three of them, none works really. Sometimes the Laptop boots with its internal graphic card, sometimes it tries to operate the GPU I d say, but it rather makes a dead graphic card bios beep or does not do anything, just keeps trying to start I guess. Am I making any idiotic mistakes? I tried to boot the laptop with the graphics card with 4 and 8 gigs of RAM, no difference. the GPUs are very simple ones, I know the Dock only supports up to 50 Watt. The video output should be working over HDMI, DVI and VGA alltogether on the cards, should it? I am happy for every help provided on my issue. Or is it really needed to read the manual to get it working? I followed some Youtube videos and their dock work, mine not until now. Would be sad if the dock itself is broken. All my three graphic cards are not for sure, maybe one but not all three. Thanks in advance!