how can I cancel my contract and giveback the modem?

If your decision is taken

https://fizz.ca/en/faq/how-do-i-unsubscribe-from-fizz-home-internet

1) Log into your account and go to Manage.
2) Click on Unsubscribe.
3) Scroll to the bottom and confirm by clicking Unsubscribe.
4) Tell us why you are unsubscribing.
5) Enter the date when you want Home Internet service to end.
6) Click Unsubscribe.
7) On the next screen, download, write down or print your return ID number and follow the instructions for returning your Wi-Fi modem.

Important: You have 15 days following the date you unsubscribe to return your Wi-Fi modem, or else you will be charged the amount of $150 (plus applicable taxes). Detailed instructions will follow.
How do I return my Wi-Fi modem?