To be honest, you probably can't duplicate your current cable experience with only a Roku device. If there are cable channels you can't do without then you'll probably need to add a cable-like service like Fubo, YoutubeTV, Philo, etc. Some have a cloud DVR service (Roku devices can't record anything). Streaming is a whole new way of doing things and I would say not everyone can adapt. Roku calls what everyone else would call an app a "channel". Don't be confused when Roku says you can get hundreds of free channels. That's misleading if not a flat-out lie and it's even more confusing when these "channels" (apps) have what would more traditionally be called channels within them.
Most of what I watch is on-demand and the "live" stuff I watch is via one of the free channels/apps most of which is just one rerun after another and I jump from one channel to another when ads start playing. There's no way to DVR this stuff to skip through the ads and they're becoming more and more prevalent and annoying.
If there's a show or channel you can't live without do your research to find out how to get it using only a Roku device. Some cable channels have an app for Roku devices, but they require a cable provider to unlock access to it. Their Roku "channel" is really just a way to watch it on a TV that doesn't have a cable box connected to it, but you still have to be paying for it via a cable provider. Some of the aforementioned streaming services can be used for validation.
In summary, don't expect to get what you have now for free.
6 cable alternatives and live TV streaming services (2022)
One last point, if that modem doesn't have a built-in router then you'll also need a router.
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