When you turn your phone on for the first time with your new Honest SIM card installed you will receive a text message with the settings to activate data on your handset. Wording will vary by phone, but click "Accept" or similar when they arrive.
If you're an iPhone user don't forget to add your MMS and Personal Hotspot (tethering) settings below.
If you don't get your settings automatically, text 'DATA' to 773000 (it's free) to have the settings sent to your device.
The Old Fashioned Way
If all else fails, you need to manually enter your settings. Go to
Settings > Mobile Data > Mobile Data Options > Mobile Data Network.
Enter as follows:
On an iPhone, you will have to enter the MMS and personal hotspot APNs manually as follows:
Username - *leave blank*
Password - *leave blank*
You may be prompted to enable LTE (4G) on the handset. If prompted to do so tap enable LTE.
Tap > settings > Mobile data > Mobile data options > Voice & data then select LTE to make sure you are getting the fastest speed possible.