Open Control Center (depending on your iPhone model, swipe down from the top right corner of the screen or up from the bottom). To turn off cellular data, tap the cellular icon so it's not ...

Turn Off Wi-Fi Assist to Prevent Data Spikes When your Wi-Fi signal is weak, your iPhone can automatically switch to cellular data to maintain a stable connection.

Regardless of whether you have an AT&T, T-Mobile or Verizon plan, your data is being tracked by default. Here's how to protect your privacy and turn it off.