I would like to have a single textbox outside the grid, and if the user types for example "ABC" all rows that on a column they have a value ABC would be displayed.

Hello Vassilis,

One option to achieve the desired functionality is using the Grid client-side API as suggested in how to filter a grid manually from external textbox? forum thread. 

Another option is to rebind the Grid and use the TextBox value as a Where clause of the Select statement as suggested in Filter RadGrid on external textbox key press Code Library project and Filter radgrid on button click forum thread.

Regards,
Peter Milchev
Progress Telerik