HDMI Connection to External Monitor

I purchased a third party HDMI USB-C adapter to connect an external monitor to my MacBook Air. It's plugged in and you can see the background graphic on what would be the home screen on my mac but you cannot see any of the windows that I have open on my mac. I have tried all of the Display setting options with no success. It will not scan for new displays. The only way the external monitor mirrors what is on my mac screen is if I Airplay my mac to my smart TV but then my family cannot use the TV.