I'm already subscribed but I'm being asked to sign in
If you've subscribed directly through The Google Play Store (Android) or The App Store (Apple), you don't need to sign in.
If you've subscribed, but you're being shown a paywall and the app's content is locked, tap "Restore Purchase" on the Subscribe page. This will unlock the app for you.
The "Sign In" function is only for subscribers who have created an account on supersimple.com