Check Screen Recording permissions
I just went through this with a Macbook Air and a Dell D6000 Dock connected over USB 3.1. Everything else plugged into the dock worked fine except for the display. Here are two things I did to get it working:
1. Install the latest DisplayLink software and reboot.
2. In System Preferences > Privacy and Security, you need to grant Screen Recording permissions to Displaylink Manager. Once I did that, the monitor appeared.
I'm not sure if #1 was even necessary, so you might try #2 first.
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