The no-app way is to enter the service menu and change it from there. Warning - an incorrect setting in here can give you a really bad experience - however the setting I am describing will be reset by a toggle on and off of "Airplane Mode" or at worst a reboot. Change other options strictly at your own risk.
Open your phone app (like you were making a phone call) and dial:
*#*#4636#*#*
If you did it correctly, on the last * a menu will appear.
Select "Device Information"
Under the "Set Preferred Network Type" pulldown, select the network type you want. My AT&T phones all show "LTE/UMTS auto (PRL)" by default. In your case you can experiment with "LTE Only".
I don't recommend playing with the other options unless you do some research on them first, except maybe the "Cell Info Refresh Rate" at the bottom of the menu as that only sets how often that specific menu will refresh the cell data it displays at the bottom, which can be useful to see if it is working correctly. I would set that to "Min 5s" so you can watch cell towers come and go for a minute after making changes to make sure things are behaving as you like.