I've been getting charged a monthly fee for 3 months, and when I try to utilize some features, I get an upgrade to Premium notice.. So, My ideal suggestion to U guys is, Why not do what Ur getting paided for??
Do you get email receipts for your subscription purchases? Search for one of them in your email account. In it, the Spotify username you're paying for is shown:
Logout of the app, and be sure to use that username to login to your account. It's the one with the Premium features.