I too saw that message for an att.net account I have. I checked "Always Trust" and, for what it's worth, the message has not shown up again.
What couldn't the AT&T rep figure out? That AT&T forgot to renew its certificate in a timely manner? Why your "Always trust" did not stick is another issue.
On a Mac since 1984.
Currently: 27" iMacs, Macbook Air, macOS 10.15.x,; iPhones, iPods and iPads galore!