Whoa, exact same issue.
Clearwater/Tampa Bay area.
I thought it was my iTunes Match not working with my new iPhone 6S Plus.
I was very close to exchanging it.
I am getting decent LTE speeds and everything else (Youtube, safari) work fine.
My phone sees all my music but refuses to load it, its like a firewall preventing it.
So, I am driving home and realized I forgot to install iHeartRadio. An app that is about 70megs, I think.
The icon just sits there like its loading and for the full 1/2 hour ride, doesn't budge.
Next, I take out my wife's iPhone 6 on my walk (while I manually load music on my 6S because Match isn't working) and notice the exact same thing.
I never considered it was an AT&T thing.
Maybe they are ninja throttling because so many new activations come out this time of year?