No, eero doesn't replace your modem. eero replaces your router.
Setup with your modem
The first eero plugs into your internet service provider’s (ISP) modem (we refer to this as the Gateway eero). If you have a modem/router combo, we suggest you place it in bridge mode. For instructions, see How do I bridge my upstream modem/router combo device?.
Additional eeros just need power from a standard outlet.