Why can't I unlock my lg g3

[AC Question]

I purchased the wrong lg g3 and its an at & t model (lg g3 d850). I got an unlock code for it from at & t (never an at&t customer) and put a t mobile sim card in to find no place to enter my unlock code. Some extra info: my phone is rooted and I tried to dial the number which was supposed to let me unlock my phone but it said a message.