Your iFly subscription lives on your account, not on a device. Signing in on a device attaches it to the account and unlocks the subscription there; detaching frees the slot for a different device. Here is how to do both, and how the slots work.
Already have an account?
Sign in; do not register a new one. A second account has no subscription on it, and it is the most common reason a paid-up pilot sees "trial expired".
Do this on the old device before you sell it or give it away, or when you have hit your device limit.
If the old device is lost or dead, sign in to your account on iflyefb.com and remove it from the device list there, or email us and we will detach it.
The licence is downloaded to the device when you sign in. iFly does not need a connection to fly, but if the licence on the device expires, is removed, or was a trial that ran out, the app has to reach our servers to fetch a new one and will say so. Connect to the internet, go to Menu › About › My Account, confirm the device is signed in to your account and the expiry date is in the future, and the message clears.