Ok!You own a metal GPU and you definitely need no legacy video patch.
Only one TB port works with this card and external displays and about special patches for the iMac 2011 you used the wrong thread. Check the first link in my signature.
You most likely want/need to reinstall Mojave to get rid of the legacy video patch.
I have reinstalled Mojave. Indeed, the mini-DP has started working. But now the audio does not work, although the patch is installed
Damn.
Sorry guys The output was configured for miniDP and not for built-in speakers
All works !
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