Tested with a Blackberry KeyOne which runs Android 7.1.1 at the moment.
In the cellular options, you can only choose between 2G and 3G, even if your provider and contract is LTE compatible.
In phone dial menu, enter Code
Wait a few seconds afterwards, until a new menu is opened (you do not see it is loading, but you have to wait a bit).
Choose the first menu entry, then choose your preferred LTE option.
After changing the preferred network type, it should connect via LTE immediately.
Unfortunately, it seems like this setting cannot be save persistantly; it disappears after reboot.
So as an another workaround, it might be a good idea to use the app 4G Switcher, which enters the digit code for you to enter the extended menu.