You can log in with your account on multiple devices (including across iOS and Android) and the progress and subscription will be carried over. Just be sure you check the My Account page in the app menu so you log in with the email you used previously. Because of this, you do not usually need to enable Family Sharing or Family Library for Simply Piano.
However, if for some reason, your subscription disconnects from your account, you will need to Restore Purchases on a device that has the same Apple ID/Google Play account as the original purchase. Alternatively, the device can have Family Sharing enabled so the subscription will still reconnect.
Currently, Google does not support Family Library for in-app purchases, so you will need to Restore Purchases on a device with the same Google Play account as the original purchase.
Need further help? Please email us by going to "Send Us Feedback" in the settings menu of the app or writing to firstname.lastname@example.org.