Yes, you can connect the Sonos Boost to your Internet box using an Ethernet cable, and it should work without the need to connect a speaker directly to the Internet box or the Sonos Boost.
The Sonos Boost is designed to create a dedicated wireless network for your Sonos speakers, improving the overall stability and performance of your Sonos system. When you connect the Sonos Boost to your Internet box with an Ethernet cable, it establishes a wired connection to your home network. This wired connection provides a solid foundation for your Sonos speakers to communicate with each other and access online music services.
You don't need to connect a speaker with a cable to the Internet box or the Sonos Boost itself for the system to function properly. As long as the Sonos Boost is connected to your home network via Ethernet, it will effectively extend the wireless range and connectivity for all your Sonos speakers, ensuring a robust and reliable Sonos setup.