Problem connecting MacBook Air to external monitor

My 13-inch 2018 MacBook Air seems to be unable to connect to external monitors, no matter what type of cable I use or what type of monitor I try to connect it to. I've even taken my computer to the Genius Bar and they were unable to diagnose the problem - they did a full reset but it's still not working (it wouldn't even connect to the monitors in the Apple Store so I know it's not an issue with the monitors I have). They said that all the ports appear clean, and they're clearly working, as when I plug in my portable external monitor, it will start to charge, but then says No Signal. My software has been updated to the most recent version my computer supports. I've tried everything I can think of, and really need to be able to use the external monitor for work. Any suggestions would be greatly appreciated!