If you are subscribed to PRO and have recently switched from an iOS to an Android device or vice versa:
You'll continue to be charged on the platform you initially subscribed to and remain to have access to the PRO features.
For example, if you signed up for PRO while on an iPhone, you'll continue being billed each year or month—depending on your subscription—through the App Store.
It doesn't matter where you initially started your subscription your PRO features stay tied to your account.
If you want to be billed through the platform of your new device:
You'll need to cancel your current subscription on the old platform, wait until it's over, and re-subscribe on the new one.
Go here to manage your Android subscription or here to manage your iOS subscription.