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 2G/3G/4G Auto, 2G/3G Auto, 3G Only or 2G Only.
3G (UMTS) or 4G (LTE) enables you to achieve higher data speeds than when using 2G (GSM). If you select 2G/3G/4G Auto, your mobile phone automatically switches between the three network modes provided you're within range of a 4G network. If you select 2G/3G Auto, your mobile phone automatically switches between the two network modes provided you're within range of a 3G network. If you select 3G Only, your mobile phone can only connect to 3G networks. If you select 2G Only, you can't achieve high data speeds.