Starting November 12, 2017 customers on T-Mobile ONE, Simple Choice North America, or legacy rate plans with a qualifying feature will receive unlimited talk, text and data with a maximum of 5GB of 4G LTE. After 5GB of high-speed data is used in Mexico and Canada (or your high-speed data allotment is reached, whichever comes first), customers will stay connected with unlimited data at Simple Global speeds (up to 128kbps for most plans or 256kbps with T-Mobile ONE Plus).
T-Mobile says that less than 1% of people used over 5GB of 4G LTE data and the change is being made 'in order to prevent usage beyond the intent of the product'.
More details at the link below...
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Source : http://www.iclarified.com/62927/tmobile-ends-unlimited-4g-lte-data-in-canada-and-mexico