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 two fingers down the display starting from the top edge of your mobile phone. Tap SETTINGS.
Tap Mobile data.
Tap Network mode.
Tap WCDMA only, GSM 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 only, you can't achieve high data speeds. 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 three network modes provided you're within range of a 3G or 4G network.