$75 Walmart Gift Card Offer NOT Received after 4 Months

I have had my AT&T PREPAID service activated with the $65 plan w/ AutoPay since November 17th, 2018 when I purchased a AT&T PREPAID phone from Walmart with the stipulation that a $75 Walmart gift card code would be texted to me after 3 months of payments. I have never received notification of this code and when I try to call AT&T about it I get bounced around to four different departments and none of them can either find my account number or the department says I must call a different number or they transfer me. I have never missed a bill (due to AutoPay) and have made no disqualifying changes to my plan which would prevent me from receiving this promotion. Where can I get my $75 gift card promotion code and why can nobody give it to me?