I have to say that I think it's just really cool how T-Mobile (and AT&T) have partnered with Project Loon in an attempt to restore some minimal service to remote areas of Puerto Rico. It's amazing that Loon has managed to:
Navigate their balloons into position
Get permission from the FCC to use band-8 frequencies in PR and
Gotten buy-in from the carriers to support the project and use their system.
Project Loon was envisioned as a way to bring cellular and Internet service to remote areas of 3rd-world countries in places like central Africa and South America. For this reason, their radio transmitters are designed to work on the 900 MHz band-8 used in most of the world, but not in N. America. In the US, this band is used for land-mobile-radio such as police, delivery services, etc. It looks like they found a small (4.6 MHz) slice of bandwidth that was unused in PR & USVI. This will allow them to transmit, at most, a 3 MHz LTE signal plus maybe a 1.4 MHz signal. (Not much bandwidth to cover the whole island. The unfortunate thing is that only phones with band-8 LTE will be able to use the signal. This leaves out some budget phones such as the Galaxy Grand Prime.