You should add the AJAX settings on each Page_Load/Page_PreRender. You cannot add AJAX settings dynamically in Page_Init event. It is too early in the page lifecycle since some of the controls taking palce in the ajax settings might not be created yet.
For more information please take a look at the Add AjaxSettings programmatically help topic.
For adding AJAX settings dynamically in content pages you can also
get the Manager instance on the page through the GetCurrent
static method - RadAjaxManager.GetCurrent(Page) .
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