RadControls for ASP.NET AJAX does not support Razor views, only the ASP.NET rendering engine is supported. Only the ASP.NET rendering engine is supported as described in the following help article: Limitations
You can find integration instructions in the help: Getting Started
In addition, have you tried by any chance our Editor for MVC
which is especially designed for MVC environment and is recommended for usage under MVC?
All the best,
the Telerik team
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