From a Home screen, touch and swipe up to display all apps.
Navigate: Settings > Network & Internet > Mobile Network .
Tap Preferred network type.
Tap the appropriate setting:
This is the preferred setting for most locations.
This setting should only be selected if experiencing service issues in locations that offer multiple network types and only GSM/UMTS is needed.
The preferred international carrier is automatically selected.
You can manually select an alternate carrier but it may require as many as 6 registration attempts with that carrier prior to successful registration.