This guide is for Classic SIMs, if you've got a Smart SIM please check out this guide
Your Honest SIM should automatically have the right data settings. But sometimes you need to set them up. It only takes a second. Here are the instructions for an Android phone:
Tap Settings
Tap 'Network and Internet' (depending on the handset, the name may vary slightly). If you can't find 'Network and Internet', use the search icon and search for Mobile Network
Tap Mobile Network
Tap Advanced
Tap Access Point Names
Tap Add on top right (the plus icon)
Set the name as Honest and the APN as gamma and leave everything else blank
Click the three dots in the top corner and 'Save'
Tap the dot next to the row that says 'Honest' to activate your APN
Restart your handset