These are the AT&T phones that will still work after the 3G shutdown
(Android Central)

AT&T thought it would be easier to list the phones that will work after the 3G shutdown. It was wrong. What you need to know AT&T has published a list of smartphones that will continue to work after it shuts down its 3G network. The company also lets users easily check if their device will be affected. AT&T has scheduled its 3G shutdown for February 2022. Carriers are moving away from 3G and AT&T is among them. Many people are potentially worried about how the shutdown may affect their phones, so AT&T published an exhaustive list of devices that will still work when the network goes dark. This is just the first of seven pages of smartpones. As expected, the list includes many of the best Android phones sold today, all separated by manufacturer. The Apple list is pretty small; iPhone 6 and up will remain good to go. Somewhat surprisingly, the Microsoft Lumia 950 is on the list, which means fans of the now-dead Windows Phone can continue on if they so choose...