Usually, LTE is enabled by default on devices. Sometimes it can be turned off when travelling internationally, troubleshooting or even simply accidentally changing the setting.
To enable LTE on an iPhone or an iPad running iOS 10.3 and later:
On Android, the settings vary slightly from one model to another and from one Android operating system to another. Below you will find the most common settings found to enable LTE.
Alternatively or if none of these options apply to you, visit the Mobility support section for self-serve emulators/simulators to explain how to enable LTE on your device.
Select your device brand and model and search for "Accessing mobile network settings" for instructions on how to enable LTE.