The experienced issues are expected because the client id's of the controls are not static and could change. When implementing such scenario the supported and recommended way is to use RadScriptBlock
and reference the controls as it is shown in the example below. Note that this approach also overcome issues when changes to the layout are made.
var textBoxFirstElement = document.getElementById("<%= rdtxtFirstName.ClientID %>");
All the best,
the Telerik team
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