I'm going to answer my own question

I called Samsung and additional cloud storage is not available for AT&T (and Verizon) branded phones. You are limited to the 15GB that comes with the device. I still don't get why Samsung allows these phone companies to bully it into removing services from its phones. This would never happen with Apple.
Anyway, I use other cloud storage for my photos/documents. But it's nice to have Samsung cloud to backup things like your settings, Wifi passwords, home-screen layout, etc. I guess I'll need to remove some clutter to make the most of the 15GB we get free.