Why does the plate of the steriliser appear rusty after less than a month of use, and how can I clean it?

What you’re seeing is not rust but mineral deposits (lime scale) from tap water. When water evaporates at high temperatures, it leaves behind these deposits. If not cleaned regularly, it can affect your steriliser’s performance. You can remove the lime scale using either lemon juice cleaning or vinegar & water cleaning. You may refer to the descaling method here or product manual for detailed instructions.