For the record, tech support sucked. She walked me through all the steps and they didn't work. She told me it was a hardware issue, and hung up. She did, however, thank me for being polite and knowledgeable.
Right before I gave up and called the computer manufacturer to demand a new ethernet adaptor, I decided to click on something else and -- wonder of wonders -- it worked! Now my internet works just fine -- even wireless.
I probably should have written down what I did. Oops.