Does Premium work without internet?
Yes. The app keeps a signed license on the device and checks it locally — without contacting the server. Premium stays available offline.
The license is cryptographically signed, so it cannot be faked or extended manually by editing a file on the phone: the app checks the signature and refuses if it does not match.
Changing the phone's clock does not extend the subscription either — the app does not rely only on the device's system time when checking the expiry.
When the internet returns, the license updates automatically: if you renewed the subscription on the website, the app picks up the new expiry at the next connection.