If anyone is still having problems seeing a USB to Ethernet connection in Network settings, here's what worked for me.
1. I'm using the Apple dongle, USB to Ethernet. OS is High Sierra 13.13.4
2. Could not see the connection in network settings.
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3. When you plug in the dongle, go to the menu bar, and click on About this Mac. Choose System Report. Scroll down to the USB section. You should see AX88772 (it's important that the dongle is plugged into the USB port on the MB). I think that's what the dongle wants in terms of the driver, which for some reason seems to have been broken in this iteration of OS.
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4. Go to the ASIX website and download the driver for AX88772. I used the latest version 2.6.0
5. Install the package. The machine may give you a message saying your security preferences are blocking it. Just go to security preferences and tell it to allow installation.
6. The machine will ask to restart. Restart it.
7. Go to network settings. You should now see the option for the USB to Ethernet connection. Connect your Ethernet cable to the dongle, and you should be in business.
Hope this helps. Just wasted 5 hours of my life over a few days trying to figure this out, and add in two hours with the ever polite, but in this case unhelpful Apple support people.
MacBook Pro, macOS High Sierra (10.13.4), USB to Ethernet Issues
Best Usb To Ethernet Adapter For Macbook Pro 2012