Simply put, the data you can use in the Basic mode is unlimited.
If you’re in the Premium mode (without subscription), the amount of premium data is limited. You could always earn premium data by completing in-app tasks. The more tasks you complete, the more data you earn. Alternatively, you could always consider purchasing a subscription plan to enjoy the unlimited Premium data.
Please note that you will be switched back to Basic mode if the remaining data is less than 50MB in Premium mode.