I believe your right about at&t , my 521 has it in the field test my 1520 doesn't. I was hoping there's a setting in the field test that could be changed.
You can go into field test mode by punching in ##3282#, click accept, then at bottom hit hamburger menu then settings. Then click on network type and select "manual". You will then have the option to select which radio bands to use - Select all the WCDMA bands and uncheck everything else. This forces 3g. I personally force 3g by selecting only band 2 WCDMA (1900) but just depends on your area.