Cellular data connection or MMS sending doesn't work on my Lumia 521 when using "Wi-Fi Calling"

If using the Wi-Fi Calling feature, check that the Wi-Fi Calling settings are not blocking the cellular data access:

  • Swipe left from the main screen to access the Wi-Fi Calling app (or tap on the live tile on the main screen) and go to settings. Ensure that the option Wi-Fi only is not selected. If you want to use the Wi-Fi Calling feature and still use cellular network when a Wi-Fi network is not available, change the setting to Wi-Fi preferred or Cellular preferred.

Check also that the data connection via cellular network is enabled:

  • Select settings > cellular and check that Data connection is set to ON.
  • Make sure that the airplane mode is not selected in settings >  airplane mode.

Note that when connected to Wi-Fi calling feature, only plain text messages (SMS) can be sent. If you want to send MMS messages, you need to disconnect from Wi-Fi calling feature. Go to Wi-Fi calling > settings and toggle Wi-Fi calling to off.