I'm already subscribed but I'm being asked to sign in
If you've subscribed directly through the App Store (Apple), you don't need to sign in. This page is only if you've created an account and subscribed through www.supersimple.com.
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.
Android & Amazon
If you subscribed through the Google Play Store, or through the Amazon Appstore, but you're being shown a paywall, try reinstalling the app. This will force Google or Amazon's system to recheck the status of your subscription.