I have the UDK/Ultimate Subscription but the app isn't recognizing I purchased it.

You need to make sure you’re logging into the app using the correct username and password. 

These steps should ensure you can access your UDK via the app:

Retrieve your username here.

Reset your password here.

Log into app using username and password from above. If the app is auto-logging you in, you’ll need to sign out first by clicking the menu button near the top left and then clicking sign-out.

If for some reason this still doesn’t work, an uninstall and reinstall of the app should correct any issues.