I have windstream as well and am experiencing the same problem.
From what I can tell, it Seems like windstream is the problem. I tethered my firestick to my phone and spotify worked fine on the firestick. Switched back to the windstream network and got the same offline screen again.
Unfortunately, no. But Windstream might be the problem. I'm not sure there's anyone I could call at Windstream, though--this is probably not a problem a lot of their technicians would be aware of. Is there a way to get Spotify and Windstream to talk to each other so they can sort it out?
To recap--The Spotify app works on my computer, but the website won't load correctly on my computer. The app works on my phone, but not my Firestick. And, as of today, I can't send the music playing on my computer app to my TV, which I could do yesterday.(The music would play even though the TV screen said "You're offline.")