Change the Default Selected Filter Operator:
https://docs.telerik.com/devtools/wpf/controls/radgridview/filtering/how-to/howto-change-the-default-selected-filter-operator
This doesn't work for boolean columns, if the property "ShouldGenerateFieldFilterEditors" of the column is set to "True".
Please see the attached example.
*** The fix for this issue will be available with the next LIB (version 2018.3.1217) expected on Monday, December 17.