"The cable for USB 10/100/1000 LAN has a self-assigned IP address and will not be able to connect to the internet."

I got this message in the Network control panel of my Macbook Air after turning off WiFi and connecting a Wavlink "plug & play" ethernet adapter. Even though it recognized the cable/adapter -- it said "connected" -- there was no internet. Wavlink tech support suggested an involved procedure involving changes in the Mac Library. The internet cable was plugged directly into the modem. Remarkably, when I instead plugged the Wavlink cable into a multiport ethernet breakout box (Netgear FS105) that was plugged into the modem, it fixed the problem! I have no idea why the indirect connection worked when the direct modem connection didn't.