Use iMac 2010Mid as a external monitor

I need to use my iMac as an external monitor.

This is my model:

Mac (27-inch, Mid 2010)

O.S. macOS High Sierra 10.13.6

I already check the compatibility. It can to be used as an external monitor.

I hava also a MacBookPro and I'd like to extend the display on iMac.

I purchased a cable to connect my MacBookPro (usb-c) to my iMac (model 2010Mid).

During my tests only once the monitor was activated as an external monitor.

I can't do it anymore. I have read and tried various solutions but the command keys f2 do not seem to react.

Tried to enable safe mode on both devices. Tried to Reset SCM on iMac. Checked keyboard configuration(CMD+F2 or CMD+fn+F2) on original keyboard.

I am desperate. it worked once and i don't know how

Could you please help me to solve the issue?

Thanks in advance