LG's field test mode is inside hidden menu. To bring up to hidden menu, you need to find a way to activate the activities.
1. Install Launcher Pro from Google Play
2. Long press on home screen
3. Select Activities
4. Scroll down to and expand com.lge.hiddenmenu, select Hidden Menu
5. Hidden Menu icon
6. Screen Capture
7. Select Prx Drx On Off to see LTE signal info
RSRP missed the minus sign.
8. 2G/3G Engineering mode is inside WCDMA-Only -> Modem Settings -> Engineering Mode
9. In mobile network setting, you do not have option to select LTE-only. If you want to do it, you can select LTE-Only -> Modem Settings -> RAT Selection -> LTE only. To restore, select Automatic instead of LTE only.
I don't know other settings purpose. And I don't suggest you can them if you don't know what it is.
Enjoy spy the network!