Finally worked after 3 visits!!!
1st Visit (Friday):
Spent 2.5 hours at the store trying to get approved, but the apple employees didn't really know how to do the upgrade program through T-Mobile, and then got declined because of Citizens system not working. Finally told them I just wanted to buy the phone, but they somehow ended up financing it through T-Mobile...
2nd Visit (Saturday):
Was at the store for 1.5 hours and iUP was working, but there was a glitch with T-Mobile so they could not get iUP to work for T-Mobile customers. Was about ready to give up at this point.
3rd Visit (Sunday):
Went back today and spent 2 hours at the store. First there was an issue with returning the phone (got an error saying returning the phone would not cancel the T-Mobile finance agreement, so had to call T-Mobile and sort that out). After that was all taken care of and they could process the return, signing up for iUP and getting approved took 10 min.
Happy finally worked out and I didn't give up. For reference, my credit sore is 775, and used my Discover card to sign up with a limit of $5600 and cash back limit of $400.