The so-called "4G" phones on ATT are outright frauds. Plain and simple. IMO, their credibility is non-existant.
As Trevalkin correctly points out, ATT frequently(and typically in many areas) cannot even supply 3G speeds. Given the "success" of ATT's HSPA+ network, it will take years for them to move to widespread LTE, regardless of what ATT's press releases claim.
This recent GB update wrecked the radio on my Atrix, and I can no longer get anything but EDGE speeds. So, be greatful for whatever you have. It's all you will get out of the Atrix and the ATT network, for years to come.