Target Display Mode problems iMac 27'' A1312

Hi all,

I recently upgraded the GPU of my iMac to an Firepro M5100 and installed Monterey using OCLP. Before TDM was working fine when the machine was running High Sierra. Others report that TDM still works on Monterey and Ventura using OCLP but for me it doesn't. When i connect my laptop and press CMD+F2 the screen goes black for a moment and then comes back to the desktop. Sometimes it stays black and my laptop initializes the second screen but then nothing happens and the screen on the iMac stays black until i press CMD+F2 again to go back to the desktop.

Does anyone else have the same experience and is there a way to fix it?

Thanks in advance!