Sprint or LG has removed the hidden menu options via dialer in the KitKat update. As of now, you'll be forced to use ZVA/ZV8 ROM or AOSP to change network setting to LTE only.
I'm on CloudyFlex, and I have a dialer menu *#*#4636#*#* under "Phone Information" I can toggle my service settings. I'm pretty sure I've gotten into that menu on other ROM's as well.