When T-Mobile first began offering cellular iPads in the fall of 2013, they included a plan that provided customers with 200MB of free LTE data each month for the entire life of the tablet.
As of May 7, 2017, Free Data for Life is no longer available for new activations. If you signed up for FDFL on or before May 6, 2017, you can keep it with your current tablet as long as you own and use your tablet with T-Mobile.
T-Mobile currently offers just one data plan, T-Mobile One, with unlimited data. T-Mobile One customers can add a tablet to their plans for $20 per month.