You can force an Android device to only use LTE, but it's generally hidden from users[1] because it can have side effects some users won't understand. Problem with using LTE only is networks that haven't rolled out VoLTE yet--you won't be able to send/receive phone calls until enabling 3G once again and if you happen to be in an area without LTE, you won't have a connection.

[1] Depending on the device it can be accessed by typing * #* #4636#* #* on the phone dial pad or I have an app (https://github.com/yareally/SignalInfo) that allows access to it as a secondary feature. Most devices nowadays also require root to access it, disable access from the dial pad and some Samsung and LG devices still bar access even with root if you still run stock firmware.