You can select which network modes (3G and/or GSM) your mobile phone can use. Your choice depends on the data speed you can achieve.
Tap More settings.
Tap Mobile networks.
Tap Network mode.
Tap GSM/WCDMA (Auto mode), GSM only or WCDMA only.
3G enables you to achieve higher data speeds than GSM. If you select GSM/WCDMA (Auto mode), your mobile phone automatically switches between the two network modes provided you're within range of a 3G 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.