Using the set_enabled() method of the NumericTextBox doesn't disable/enable the TextBox properly:

<telerik:RadCheckBox runat="server" ID="CheckBox1" OnClientCheckedChanged="onClientCheckedChanged" Text="Enable/Disable" AutoPostBack="false" />
<telerik:RadNumericTextBox ID="RadNumericTextBox1" runat="server" />  

function onClientCheckedChanged(sender, args) {
    let numericTextBox = $find("<%= RadNumericTextBox1.ClientID %>");
    let isChecked = args.get_checked();

    numericTextBox.set_enabled(isChecked);
}