Unfortunately Mint Mobile/T-mobile does limit the number of free trials per device per year. In their terms and conditions I read over 3 years ago.
A valid email, telephone number, mailing address, and credit card are required to redeem a Mint Trial Plan, which may be requested through the Mint Mobile app. Only one free Trial Plan per customer.
Source: https://www.mintmobile.com/plan-terms-and-conditions/