A subscription belongs to the platform where you bought it: App Store, Google Play or web. They are separate and do not transfer.
If you bought on web and now use the phone app, your Premium is recognised on your account, but you still manage and cancel it on web. The mobile apps do not show web prices or checkout links.
To move from one platform to another, cancel on the old platform, wait for the paid period to end, then subscribe on the new one. Do not subscribe twice: the app will block a second checkout while you are already Premium.
See the price ladder for how pricing works on each platform.