I just received the same message today on a line and GL-X750v2 device that is six months old
This from a text from ATT:
Hi, it’s AT&T. We’re saying goodbye to our 3G network Feb. 22, 2022, and bringing you access to our upgraded network. To keep using your AT&T service, your device on a line ending in 3793 needs to be replaced or your line will be disconnected about one month after shutdown.


I just logged in and am now applying the 3.211 update.