In order to apply the custom render in RadRangeSelector, you can use the following code snippet: 

RangeSelectorViewElement rangeSelectorChartElement = this.radRangeSelector1.RangeSelectorElement.AssociatedElement as RangeSelectorViewElement;
rangeSelectorChartElement.CreateRenderer += ChartElement_CreateRenderer;

private void ChartElement_CreateRenderer(object sender, ChartViewCreateRendererEventArgs e)
{
    e.Renderer = new CustomCartesianRenderer((CartesianArea)e.Area);
}