Do you get email receipts for your subscription purchases? Search for one of them in your email account. In them, the Spotify username you're paying for is shown:
Username/ID: xxxxxxx
Logout of the app, and be sure to use that username to login to your account. It's the one with the Premium features.
You can login with that username to the website to check the status of your subscription here:
Hey there you, Yeah, you! 😁 Welcome - we're glad you joined the Spotify Community! While you here, let's have a fun game…
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