I have ordered a replacement air card from eBay and it should arrive within a couple of days. It is a used one, and I hope I can get it to work simply by transferring the SIM card from the old one. The cost from eBay is $59, whereas I would have had to pay approx $115 to cancel my contract with AT&T.
So I should be back online soon, using my new/used air card. My neighbor has generously allowed me to use her mi fi to research and order the replacement card.