4G is activated by default in all 4G SIM Cards and You simply need a LTE compatible device and be in an area with 4G coverage.
To turn on 4G on your device please follow the below procedure.
Enabling 4G on iPhone
Go to Settings -> Cellular -> Cellular Data Options -> Voice & Data and select LTE
Enabling 4G on Android
Go to setting -> Connections -> Mobile Networks -> Network Mode and select LTE–2G–3G (auto connect)