If you frequently run out of high-speed data, you can switch to a plan with a higher "high-speed" threshold, such as AT&T Premium 2.0, which offers unlimited high-speed data that doesn't slow down based on usage. AT&T Prepaid
For most current unlimited plans, you don't "buy" more mobile data because it's already unlimited, though speeds may slow after a certain threshold. However, you can buy extra . buy more data at&t
Prepaid customers can purchase data "Top-Ups" if they run out before their renewal date. If you frequently run out of high-speed data,