I struggled a bit with getting LTE up and running on stock (rooted) 4.4. Even after editing the build.prop with the correct values, the Network Mode always reverted to GSM after a reboot.
The solution that I eventually stumbled across involved using an app like
SQLite Editor to manually edit the settings DB.
- Launch SQLite Editor.
- Apps > Settings Storage > settings.db > global
- Change preferred_network_mode to 10
- Reboot
I believe this gets around the need for a factory reset in some situations.