WiFi Assist is a feature on iOS that automatically uses your cellular data plan when WiFi is too slow. If your data plan is limited with only a few MB or GB, you may consider turning WiFi Assist off.
  1. Open the Settings app.
  2. Tap Cellular.
  3. Scroll down to the bottom of the screen and switch the Wi-Fi Assist toggle to Off.

Also see 

How do I only use Wi-Fi and not my limited cellular data plan when using the app?