I subscribed on the App's Subscription Website (NOT within the App)
You can use your same username and password you registered with on the Subscription Website to access the app to login to multiple devices.
I Subscribed within the App (and NOT on the App's Subscription Website)
In order to access your account on multiple devices, you must be signed in to your Apple iTunes Account or Google Play account on the same device you are wanting to access the app from. Active subscriptions are tied to an paid account on your Apple iTunes or Google Play account status.
You cannot switch between an Apple and Google Play account.