With the release of iOS 8, subscribers using iPhone 5c, 5s, or higher can upgrade to iOS 8 and enable LTE on their iPhone. As a default, LTE/4G will be disabled. The subscriber needs to manually enable it.
Below are the basic step by step instructions on how to turn on LTE.

Enabling LTE/ 4G on the iPhone

Step 1: Go to home screen
Step 2: Open settings
Step 3: Select cellular
Step 4: Select enable LTE switch
Step 5: Select enable LTE
Step 6: LTE now enabled

And then you will need to update your APN settings as follows in the respective columns.

APN – data.lycamobile.co.uk

Username:

Password:

MMS APN – data.lycamobile.co.uk

MMSC – http://lyca.mmsmvno.com/mms/wapenc

iPhone LTE/ 4G Setup
Then restart your iPhone and LTE will start working.