Reward Card

I purchased 2 ATT Prepaid Phones  at Walmart which had an ATT $75 reward for each line after 3 monthly payment was made (3-5 days). I purchased the phones on November 10, 2018. Today's date is January 15th, 2019. I have already made 3 payments (Nov 10, 2018;  Dec 9, 2018; Jan 8, 2019).  All of the payments were autopay except for the first one which was taken from my credit card. The 5th day would have been  on the 13th of January 2019.  I haven't received  any message with the codes needed to activate the offer.  I  have called ATT 3 times and they seemingly don't know what to do for me or they don't recognize my number, etc. Please assist me on how to  get my reward cards for the two phones.