AT&T Next Trade In

So I have a problem, I upgraded two of my devices and got the iPhone XS Max for each of the two lines I upgraded. I printed off the label AT&T emailed me and sent both phones in two separate boxes. Both tracking numbers thru their "SmartLabel Tracking" system shows it was delivered to the return center on 9/28/18. I got one text saying one of the devices was received in good condition and that one is all set, but I never got a text about the other phone being received. Yesterday I got a text from AT&T saying if they don't receive my old device they're going to add $499.50 to my next wireless bill. I have tried calling Customer support probably 3x to resolve this issue but none of them have a CLUE on how to help me with this. The tracking shows it was delivered over 11 days ago but they still texted me yesterday saying i'll be charged if they don't get it. What should I do?