The reason for the experienced issue is that the OnDataBound event is never reached, since this event fires if you have specified a DataSource or DataSourceID of the RadComboBox. Therefore, my suggestion would be to use another event of the RadComboBox - OnLoad for example.
In addition, I noticed that you attempt to persist one "Select" item in the RadComboBox. Please keep in mind that with the currently used approach for populating the combo with data, this item would be overwritten and won't get displayed. Thefore, I would suggest you to use the DefaultItem of the RadComobBox, or the EmptyMessage property to specify the desired item. Here is a video, demonstrating the behavior at my end, using the OnLoad event and the DefaultItem of the RadComboBox. Please give it a try at your end and let us know if the issue still persist.
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