should disable the skin for the combo and render it as simple select element. In addition, do you use a FormDecorator? If you have FormDecorator, your combo will not be rendered as a simple html element.
This what you can try is to use RenderingMode="simple", remove FormDecorator and try to style the output select to look like a RadComboBox with MetroTouch skin. Note that it cannot be styled exactly as the combo but at least basic look can be achieved.
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