Search in the Play Store for "Network" by Philipp Mangelow. It is actually a shortcut to the phone's built in diagnostics mode. It doesn't work for stock or ROMs with stock kernels, but I have used it to put the phone into LTE-only mode for testing. You will see both GSM and CDMA settings, regardless of which one your phone is. Try them all, sometimes the setting is mislabeled (its a bug with the ROM, not the app).
So in short, even if your ROM doesn't support LTE-only, you have a good chance of this feature with this app.