https://techcrunch.com/2017/10/20/apple-and-att-activate-provisional-lte-band-8-to-give-iphone-users-in-puerto-rico-cellular-service/

Apple has been working with AT&T to extend and activate cell service for users in Puerto Rico. To improve what is a terrible connectivity situation there, it’s going to enable a provisional band of LTE that has been recently approved, but not activated in the US and Puerto Rico, where it has not been licensed.

Because LTE is not licensed on Band 8, many phones have not yet been equipped for use with it, though some newer radios are able to connect to it and use it. Apple devices from the iPhone 5c and up and running iOS 10 or higher can download a carrier setting and begin using it right away.