When the grid has a horizontal scroll bar displayed, we can't scroll the table horizontally using keyboard only.  If testing using the Siteimprove Accessibility Checker, we will get the issue "Scrollable element is not keyboard accessible".

The reason is that when the RadGrid is rendered, the horizontal scroll bar is render as a separate <div> out of the data table, and the horizontal scroll bar doesn’t have a tabindex, so there’s no way to put focus on it using keyboard.  Can you add tabindex="0" to the horizontal scroll bar?