Hi @eabacchini, welcome to the Community!
I understand you want to cancel your subscription. No problem, our customer support will fix it for you! Dial 0800-0092 and request a cancelation of your Thuis contract.
You have a term of notice of one month, which means your subscription will be cancelled on March 12th 2021. Once your subscription’s been cancelled, all you have to do is return the hardware: the modem, TV Box(es), remote control and if possible please return the cables as well. You’ll receive an e-mail with more info on this shortly hereafter. If you want more information about returning your equipment, please take a gander in this topic: How to return the equipment.
Thanks in advance! It's always unfortunate to see someone leave, just know that should you ever change your mind: you're always more than welcome to rejoin the club! 
Enjoy your new Fiber-subscription and have an amazing weekend!