From the Verizon S8 support pages :
Samsung Galaxy S8 / S8+
From a Home screen, touch and swipe up or down to display all apps.
Note These instructions apply to Standard mode only.
From a Home screen, navigate: Settings Settings icon > 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 experiencing service issues in locations that offer multiple network types and only LTE/GSM/UMTS is needed.
Note The preferred international carrier is automatically selected.
Note You can manually select an alternate carrier but it may require as many as 6 registration attempts with that carrier prior to successful registration.