I can't find APN in my iphone settings

I have an iphone 4 and changed networks from orange to giffgaff, but for my old number to work with the new sim card I have to find APN on my phone settings and I cant find it anywhere, and I need this to be able to active the old number with the new sim card. Is there a way to do this without using APN, or how can I get the APN section in my phone settings? My phone is not letting me contact anyone through text or calls that doesn't have an iphone, but it works normally with people who have iphones