Welcome to the community!
Firstly, to clarify, is your router also a modem? If it isn't, it would still need a modem to connect to the internet. While some modems offer router functionality, not all routers can be used as modems.
That said, I don't think it's possible to use your own modem. You will have to use the Fido modem for Internet access. You should note that the advertised price for Internet already includes the cost of the modem rental (see here). You can, however, use your own router in lieu of the modem's built-in functionality. You would just need to switch your modem to bridge-mode.
Hope this helps 😀