As explained in the help
you cannot change the Text value of the RadInput control when its ReadOnly property is set to "true". In short, if you intend to modify the value of the control you should not use ReadOnly. If you want to be able to change the Text value (via client or server code) and at the same time prevent the user from doing so, you can use the Enabled="false" property. I hope this helps.
the Telerik team