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 networks.
Tap Network Mode.
Tap WCDMA only, GSM only, WCDMA (preferred)/GSM or LTE (preferred)/WCDMA/GSM.
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 WCDMA (preferred)/GSM, your mobile phone automatically switches between 3G and 2G provided you're within range of a 3G network. If you select LTE (preferred)/WCDMA/GSM, your mobile phone automatically switches between the two network modes provided you're within range of a 3G or 4G network.