Thank you for contacting Telerik support.
The property render mode auto run a device detection by used browser not by the width of the window. So if you set this property to auto, the RadMenu will render in mobile render mode on mobile devices. In browsers on PC it will be rendered in lightweight render mode even if you increase the window. Changing the render mode automatically on the PC is not supported scenario and it is necessary to add additional code. We are sending you an exemplar solution how you can achieve it. The breakpoints that it set in the solution is 1000 pixels. We hope this will resolve your problem.
Do not hesitate to contact us if you have other questions.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.