Problems switching network mode on Android 5.0

[AC Question]

I have an LG L33G Sunset LTE Android Straight Talk 5 MP Prepaid Smartphone. It is Android 5.0 and uses Lollipop OS. I need to change network mode from 4G LTE either CDMA or GSM so that I can make calls and receive texts. However, I have been unable to do this through the settings menu. When I click setting and network settings it only brings up one network and when I click on it it only darkens. It will not allow me to do anything else and does not open any new menus. I have also tried dialing *#*#3646#*#* and *#*#3646633#*#* which did not work. I have tried apps though Google Play and even calling Straight Talk. They could only troubleshoot the phone and none of the apps worked. Please help. I really need to change my network mode so that I can be able to make calls and receive texts.