Select the giffgaff APN settings from the pop-up menu that will appear on your device when you insert the giffgaff SIM card. If nothing happens or the menu does not appear, you will have to configure the mobile data connection manually.
Go to Settings —> More —> Mobile networks. These steps may vary slightly depending on the Android version.
Activate data roaming
Activate data roaming and then go to Access Point Names (APN).
Click on the (+) symbol in the upper menu and enter the following data.
Rest of fields leave blank.
Save APN settings through the top menu.
Select the new giffgaff APN created and exit the configuration menu. Disconnect the WIFI and you are ready to go!