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 networks.
Tap Network mode.
Tap 3G only, 2G only, 3G (preferred)/2G or 4G (preferred)/3G/2G.
3G (UMTS) or 4G (LTE) enables you to achieve higher data speeds than when using 2G (GSM). If you select 2G only or 3G only, you can't achieve higher data speeds than what you've selected. If you select 3G (preferred)/2G, your mobile phone automatically switches between 3G and 2G provided you're within range of a 3G network. If you select 4G (preferred)/3G/2G, your mobile phone automatically switches between the three network modes provided you're within range of a 3G or 4G network.