Of the option exists on non branded versions then AT&T had removed this option (which they are notorious for doing). Your only option is to manually switch. To do this go to phone, dialer and type ##3282#. Click "Accept" and then click the "..." (3 dots and be careful not to click anything above it) at the bottom and "Settings". Change "Network type" to manual and then under "Supported bands" uncheck the bands you don't want enabled. When you hit the back key, toggle the ENS boot option to off (if you don't do this every time you put your phone on airplane mode or reboot everything resets) and exit. Reboot your phone.