In general, you are able to use the Kendo Validator to display model errors. This works successfully with the NumericTextBox, ComboBox, DropDownList and the rest of the widgets from the inputs group.
However, after further investigation, I can confirm that there is an issue with the MaskedTextBox TagHelper, which prevents the data-* attributes from being rendered on the generated input element. Thus, the validation messages are not displayed successfully. I have logged this as a bug in the official Kendo UI GitHub repository. You can start tracking the progress that we make on it from issue #4415
As a workaround, you could either add the data-val and data-val-required attributes manually or use the HtmlHelper for rendering the widget.
Also, as a small token of gratitude for helping us discover this issue, I have updated your Telerik Points accordingly.
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