The problem is with the ROM your using. As the stock LG ROMs are assigned to your phone's IMEI, than, if you're using a ROM version (...20a, ...20b, or ...20c) that has not been assigned to your IMEI, you will not be able to access you LTE RAT Selection.
I had this same problem way back, when the first European branded KitKats starting showing up, and I flashed the Spanish and German one, but, each time I ended up with no LTE.
Same problem sometimes happens with custom ROMs, like CM or PA. You have to ask the Dev of the OP forum from which your ROM is.
Sometimes a simple kernel change might help.