I was in Mexico last week and using my AT&T phone, which is on one of the 15GB plans that was supposed to include free roaming in Mexico. I stopped into a retail location right before the trip to ask if the feature needed to be activated, the employees pulled up my account and determined that the feature was automatically active on eligible accounts now (like mine) and no need to activate it manually.
Shortly after stepping off the plane in Mexico, I began accruing data usage charges at a rate of $2,099.20/gigabyte. A chat to AT&T has (supposedly) removed these charges and gotten the feature activated on my account, but...
1) be wary of traveling outside the US with AT&T, even if it's supposed to be included in your plan, and
2) $2,100/gigabyte? Seriously? How is that a thing in 2017? Yes there are packages, but failure to choose a package results in downright abusive rates...