Hi and welcome,
If you are on Cubase Pro, enable Control Room (Studio > Audio Connections), add Monitor 1 bus and route this one to your main output. Then set your Stereo Out from the Audio Connections > outputs to Not Connected. All Preview outputs are routed via Monitor 1.
Or if Control Room is not enabled (or you are not on Cubase Pro), Right-click to the Stereo Out in the Audio Connections > outputs and enable it as “Main Mix”.