If you don't see the enable/disable LTE option in your iPhone 5 settings, you won't be able to receive a 4G signal.
To fix this, you need to update your carrier settings, which is dead easy - just follow these steps:
1. Make sure your iPhone 5 has an internet connection
2. Tap Settings then General and then About. If an update is available, a prompt will appear as below:
3. Tap OK to install the update.
4. Hold the Sleep/Wake button down until you see Slide to power off
5. Slide to power off
6. After the device powers off, hold the Sleep/Wake button to turn your phone back on