Generally, such Menus in mobile scenarios should be on the top of the page and should expand downwards. That is why by design the Menu with RenderMode="Mobile" expands always downwards.
Nevertheless, we have submitted a feature request
in the Telerik Ajax Feedback and Ideas portal where you can comment on the scope of the feature, how it should be controlled, example business cases, everything you find relevant. This will let the community evaluate, comment and vote on the idea.
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