Help getting started with Roku devices, including setup, connecting your device to your TV, linking a Roku device to your account at my.roku.com/link, adding channels, subscribing to services, and more.
You obviously can't use a physical cable since standalone Roku's don't have tuners, and even a RokuTV can't handle encrypted tuned channels. There are various Internet-based 'cable' alternatives, although I'm not sure just how many are available in Canada. Using them could cause issues with any data caps. Some cable services may offer their content through various apps, although not all offer it through Roku, but only phone/tablets and maybe computers.
As @andyross mentioned, there are cable alternatives available for Roku devices. I suspect that some/most might be available in Canada. You can start by checking for YouTube TV and Sling TV. Both offer a number of "cable" channels. For more specific content, there's CBS All Access, HBO is available, and I'm sure I've overlooked more. If you have Amazon Prime Video, they also offer the ability to subscribe to other providers within their system.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Ultra (4800), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.