On normal settings you cannot lock to LTE only but with special menu you can do that. Remember, if you select LTE only you cannot receive calls without VoLTE so we do not recommended select that. 

This working mostly common phones like Nexus, Motorola, Sony, LG, OnePlus etc... Code not working without root on Samsung.

1. Open Phone App and call to *#*#4636#*#*.

2. Select Phone information.

3. Select then what network you like force. If you select LTE only, you will not receive calls because mostly providers network not have yet VoLTE!

WCDMA preferred -> 2G/3G, 3G prefered

GSM only -> 2G only

WCDMA only -> 3G only

GSM auto (PRL) -> 2G

CDMA auto (PRL) -> 2G CDMA

Global auto -> Automatic

LTE/CDMA auto -> 4G, 2G CDMA

LTE/UMTS auto -> 2G/3G/4G - default on common devices

LTE/CDMA/UMTS auto (PRL) - default on Nexus 5X

LTE Only -> 4G only. You wil not receive calls if you select this without VoLTE!