try to set border individually like below
BorderRAnge.Borders[Excel.XlBordersIndex.xlEdgeBottom]
BorderRAnge.Borders[Excel.XlBordersIndex.xlEdgeRight]
BorderRAnge.Borders[Excel.XlBordersIndex.xlEdgeLeft]
BorderRAnge.Borders[Excel.XlBordersIndex.xlEdgeTop]
hope this helps