Hi. So you have purchased a giffgaff sim from Tesco. Is that correct?
Did you report your old giffgaff sim as lost /stolen? If so, you will be sent out a new sim automatically. This is the only sim you can use.
If you didn't report your sim as lost /stolen then all you need to do is carry out a sim swap.
Use the link below for information about sim swaps including an activation link.
Hello hi why is mobile data too slow last 2 days cant use anything