I recently purchased an 7Plus to replace a 6s. I bought it from the Boost website, so everything should work. After some trouble, I finally got through the activation pages of your site. It said to give it 15 minutes to activate. It hasn't. It had a "troubleshooting" link, but that just sent me to the Apple website. I don't think they can help getting it activated with Boost.
It's showing Sprint at the top of the phone. It won't make or receive calls or text messages.