Google Play purchases: all your purchases are bound to your Google Account. You will have access to the Premium content as long as you're using the same account as before.
For now, there is no way to move purchases from Android to iOS (and vice versa).
Website purchases: if you subscribed to the app using the website, you can move the membership between three devices using a code given.
Note: the devices can't sync progress with each other.
If you keep experiencing issues, please send us an email at firstname.lastname@example.org, and we'll help you restore the purchases!