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: 24" M1 iMac, M2 Pro Mac mini with 27" BenQ monitor, M2 Macbook Air, MacOS 14.x; iPhones, iPods (yes, still) and iPads.
| 
