OpenXML C# Excel datatype

I am dumping the content of a multidim array into excel via openxml.
It works but when I open the excel file, the numbers are stored as text which displays the little green triangle in the upper left of the cell.

Here is how I add the rows/cells and assign the array value.

  int arrayCounterID = 2;
                for (uint j = 2; j < rplUpdatedData.GetLength(0); j++)
                { 
                    row = new Row() { RowIndex = 7 + j };
                    uint rowNumber = 7+j;
                    
                    for (int i = 0; i < 15; i++)
                    {

                        Cell cell = new Cell()  
                        {
                            CellReference = data[i] + rowNumber.ToString(),                 
                            DataType = CellValues.String,
                            CellValue = new CellValue(rplUpdatedData[j, arrayCellID[i]].ToString()),
                            StyleIndex = 4
                        };

                        row.Append(cell);
                        arrayCounterID++;
                    }             
                sheetData.AppendChild(row);

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I add an if statement to test the array value and set the datatype and value.

int arrayCounterID = 2;
for (uint j = 0; j < rplUpdatedData.GetLength(0); j++)
{ 
    row = new Row() { RowIndex = 7 + j };
    uint rowNumber = 7+j;

    for (int i = 0; i < 15; i++)
    {

	Cell cell = new Cell()  
	{ CellReference = data[i] + rowNumber.ToString(), StyleIndex = 4 };

	double isNum = 0;

	if (Double.TryParse(rplUpdatedData[j, arrayCellID[i]].ToString(), out isNum))
	{
	    cell.DataType = new EnumValue<CellValues>(CellValues.Number);
	    cell.CellValue = new CellValue(rplUpdatedData[j, arrayCellID[i]].ToString());
	}
	else
	{
	    cell.DataType = new EnumValue<CellValues>(CellValues.SharedString);
	    cell.CellValue = new CellValue(rplUpdatedData[j, arrayCellID[i]].ToString());
	}                    
	
	row.Append(cell);
	arrayCounterID++;
    }

sheetData.AppendChild(row);
}

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This creates the excel file and the XML source looks ok but the Excel file appears corrupt and will not open.
Any thought on my approach?