You can force the switch to 4G (on a compatible phone and network) by doing the following:
1. End all data sessions:
Android: Close all open applications by holding down the home button until all the running apps are shown then selecting the pie icon at the bottom left of your screen. Select "End all".
iOS: Double click the home button or swipe / hold up from bottom of screen, then swipe the applications away from bottom to top to close.
2. Put device into Airplane mode for about 90 seconds
3. Power the device off then on
These steps assume that you have enabled LTE or 4G mobile data on your device previously.