if you had a subscription, you did not buy the extension, you were entitled to use it, (and all of the other extensions) for the duration of the subscription.
if you purchased the extension, then its yours, to keep, forever.
if however you were using the extension as part of the subscription, and you owned it beforehand, you may well be using a different activation serial number.
if this is the case, log in to your account and check your purchase history, and your serial number details will be there.