Are you sure? I have a 2009 27” iMac that by the internet said I was limited to OS 10.12.6 and wanted to use it as a second screen. I contacted Apple Support and after I told them all the apple products I had they gave me a link to upgrade to 10.13.6 and I now can use the iMac as a screen only. I was told everything after OS 10.13.6 has that capability. Your machine should be way ahead of that. It could just be your cable you are using. You would need a Thunderbolt 3 or 4. I had to use a third party adaptor in the back of the computer to be able to use a USB C. Good luck.