Not Able to sign into HBO Go

I’m trying to access HBO GO via mobile app on my iPhone. It’s been awhile since I last used it. When I tap the app, and try to sign in, I am asked for my provider. I picked Xfinity. I’m taken to an Xfinity page that says I need to add HBO to my plan. I have had HBO as part of my plan the entire time I’ve been in Xfinity customer. I deleted the HBO Go app and re-downloaded with no luck. Why can’t I sign in?