Once you have logged in to https://mail.gen.cam.ac.uk/roundcube/ using your departmental username & password, go to personal settings (top right corner)
Then navigate to the "Folders" panel
A list of available folders should become visible to the left hand side of the window. Find the "shared" > "reception" > "inbox" folder and tick the box next to it.
You may also want to set up RoundCube so that mail appears to come from reception. To do this go to "Identities"
Then add a new identity with the add button:
And fill out the new identity. Don't forget to save it!