Unable to unlock an iPhone even after the request is "approved"

My device unlock request was approved and I got a confirmation email. However when I put in my sim card (a UK sim), it does not work. It comes up with the same message as before, saying the sim card is not accepted by the carrier's activation policy.. when I have had confirmation from AT&T that it's unlocked.
My partner gifted me this iPhone 6s, which is no longer covered by his contract. Please help.