It took three weeks and doing an online chat with AT&T support every 3-5 days to get the Unlimited data back, but we were finally successful. I am convinced that they try to wear you down so that you give up. DON'T give up. keep on them and insist on an ETA of when you will hear back from their escalation point. When that day comes, contact support and push for resolution. If it is not resolved at that moment, get an ETA and when that day comes, contact support and push for resolution. Repeat until you get your unlimited data plan. Do not give up!!
Also, I recommend using their online chat as it is easy to copy/paste the text of the sessions.
This phone upgrade was a completely frustrating mess!
Good luck y'all!