T-Mo is simply trying to use this as an excuse to "bump" everyone up to the next most expensive data plan.
Mine works as it supposed to on the $19.99 plan, here is how I did it.
Get on the phone (not in the store) and call customer service, ask for a supervisor, be prepared to wait.
Be patient, courteous, tell them you like T-Mo (do not threaten to switch carriers) and surely there must be a simple mistake.
The super will then be eager to help you as they hardly ever get courteous patient customers.
It worked for me it will work for you.