For subscriptions purchased through Google Play Store (Android) and App Store (iOS and macOS), there is an option to restore the purchase that can come in handy if you don’t see the Premium Subscription activated on your device anymore.
To use it, on any of the platforms, go to the Get Premium purchase screen from the main page of the app and then click on the Restore Purchases line there at the bottom. The system will prompt you to put in your AppleID/Google credentials. Make sure to put credentials from the same account that was used to purchase a subscription.
Please note, that this can be done only on the type of device you’ve originally purchased from - e.g., if you have the subscription on iOS App Store you should use an iOS device for Restore Purchases option as well.
If you are looking to just add another device to your subscription you will need to just log in with your ClearVPN account. Please find more information on that in your article here.