From a Home screen, navigate: Apps icon > Settings > Network & Internet.
Tap Mobile network.
Tap Preferred network type.
Tap the appropriate setting:
When LTE/GSM/UMTS is selected, if presented:
Tap one of the following options:
When selecting Manual, tap the preferred network.
The preferred global 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.