I am having the same troubles, i am using a 2017 Macbook Air with a USB 3.0 to HDMI Adapter. The second screen is just showing up as black, the monitor seems to register that it is connected but the Computer doesnt recognize that a second screen is available and there are no display options.
The problem started when updating to 10.13.4 and i haven't been able to get the second screen to work since. I was hopeful that the 10.14 update would fix that issue but it seems to be more of the same.
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