The latest date by which Cablecom must have received the letter in your case is Jan 31, 2018. But you can cancel anytime before that, including as early as today or, looking back, when you signed.

When cancelling a bit early you can use ordinary mail, requesting confirmation to save the additional cost of registered mail. If you don't get a confirmation within, say, one week you could call or send another one, perhaps this time using registered mail.

However, once cancelled it may be difficult or impossible to undo the cancellation and continue the contract. So cancelling super-early may not always be the best option.