Had the same problem with iMac 2017 running Monterey and 2 external 27"Dell monitors, could not get iMac to recognise them. My cables are Thunderbolt 3 to Display port x2 This worked for me... Forget the system pref, turn on your Mac with external monitor(s) linked, then unplug cable from back of external monitor and put back, screen should go black and then connect. Thats it! My monitors stay connected if I put iMac to sleep, but if I restart I need to do the cables trick again.