If a device will not let you save a new APN, try editing one of the existing APNs, eg o2.
The name of the APN does not matter if you can get one of the existing APNs to save the other giffgaff settings and then select it.
However, if the device was originally sold for a specific network, or has been configured for employees of a particular company, it might possibly be that the device has some kind of protection to prevent APN editing.
When I click on the http address, it takes me straight through to the VirginMedia SuperHub (this is my internet provider). I am unable click on the 'Wizard' tab because I can not find it - and therefore I am unable to establish a successful network connection. Can you help me please? Julie
@julief114 That happened because you were connected to your VirginMedia router instead of the mifi, you need to disconect from the Virgin one first and then connect to the mifi, then you should be able to access the settings. I don't own one of these so can't be specific but the guide in the OP looks quite clear. If you can't stop your device connecting to the Virgin router maybe turn it off temporarily or just disable the wifi on the router.
EDIT:- Ooops, just noticed the date on your post was from years ago so I expect you've sorted the problem by now