HTC One M8 for Windows - Manual (Page 109)

Changing the network your phone connects to
By default, your phone is set to Global mode. It automatically searches for
the appropriate network in your location (CDMA or GSM) and then
connects to it.
When roaming in countries outside of North America where both CDMA
and GSM networks are used, you can choose what network to connect to.
1. Swipe down from the top edge of the screen with one finger.
2. Tap All settings > cellular+SIM.
3. Tap SIM settings.
4. Tap the Mode selection box, and then select LTE and CDMA or LTE,
GSM, or UMTS.
Phone calls 109