If a Subscription is cancelled in the backend, then it is not possible to re-activate it since cancelled subscriptions are set to remain cancelled. However, if the Subscription is active and the payment is in Pending state, then it is possible to re-trigger the payment process.
Simply change the Payment Due Date by going to the Subscription Edit page and clicking on that date. This will change the payment date to the chosen date and then billing cycles are offset based on that new date.
* A point to note here is that you are essentially “adding free subscription time” if you are moving payment date to a later date.