developed a widget to toggle (preferred) network types, e.g., 2G, 3G, 2G/3G, LTE, on Android 5.0/5.1 (Lollipop) and Android 6.0 (Marshmallow). It requires root.
http://forum.xda-developers.com/android/apps-games/widget-toggle-network-type-5-0-xda-t2945406