Navigate: Settings > Connections > Mobile networks.
Tap Network mode.
Tap the appropriate setting:
Global: This is the preferred setting for most locations.
LTE/CDMA: This setting should only be selected if experiencing service issues in locations that offer multiple network types and only LTE/CDMA is needed.
LTE/GSM/UMTS: This setting should only be selected if experience service issues in locations that offer multiple network types and only LTE/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.