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!
LTE/WCDMA -> 3G/4G
...