You can select which network modes your mobile phone can use, eg. 3G or GSM. Your choice depends on the data speed you can achieve.
Slide your finger down the display starting from the top edge of your mobile phone. Tap the settings icon.
Tap More settings.
Tap Mobile networks.
Tap Network mode.
Tap LTE/GSM/WCDMA (Auto mode), GSM only or WCDMA only.
3G (UMTS) or 4G (LTE) enables you to achieve higher data speeds than when using 2G (GSM). If you select LTE/GSM/WCDMA (Auto mode), your mobile phone automatically switches between the two network modes provided you're within range of a 3G or 4G network. If you select GSM only, you can't achieve high data speeds. If you select WCDMA only, your mobile phone can only connect to 3G networks.