You can select which network modes your mobile phone can use, eg. 3G or GSM. Your choice determines the data speeds you can achieve.
Slide your finger down the display starting from the top edge of your mobile phone. Tap the settings icon.
Tap Mobile data.
Tap Network mode.
Tap WCDMA only, GSM / WCDMA auto or GSM / WCDMA / LTE auto.
3G (UMTS) or 4G (LTE) enables you to achieve higher data speeds than when using 2G (GSM). If you select WCDMA only, your mobile phone can only connect to 3G networks. If you select GSM / WCDMA auto, your mobile phone automatically switches between 3G and 2G provided you're within range of a 3G network. If you select GSM / WCDMA / LTE auto, your mobile phone automatically switches between the two network modes provided you're within range of a 3G or 4G network.
Tap the Home key the necessary number of times to return to the home screen.