I’m planning to buy a router and modem and put OpenWrt on it for maximum control and privacy. While I could get a router with an integrated DSL modem, the previous tenants had cable internet so I’m not sure if the DSL connection even works and DSL internet is also more expensive (at least where I live). Fiber optic is not available. The problem is, there is apparently no open firmware for cable modems so I would have to buy a standalone router and a standalone cable modem. I would put OpenWrt on the router and use whatever proprietary firmware came with the modem.

So my question is:
Can a standalone modem that doesn’t do routing, spy on you?
If yes a rough explanation how would be appreciated.

It seems that modem and router are used interchangeably on the internet (probably because they are mostly combined) so it is really hard to find any information on modems. Here are both Wikipedia articles for reference:
https://en.wikipedia.org/wiki/Modem
https://en.wikipedia.org/wiki/Router_(computing)