Custom sorting in list is not working - SAP UI5

I have tried to implement custom sorting and grouping on a list items, however no call is being made to the custom comparator function.
used sap.ui.model.sorter for the same.
find below code snippets for implementation, there is no call made to "customComparator" function.

already referred to the following blog:referred blog for custom sorting implementation

any help would be highly appreciated