How can I render added grid view row in render control?

Question

I have a html file that contains gridview

<div id="divAveCostUtil" runat="server" class="row d-flex justify-content-center" style="border:1px solid #00AEC0;  width:99%; margin:10px;  border-radius:23px;">
             <div class="d-flex justify-content-center">
             <h4 style="color: #00AEC0; padding-top:10px">Average Cost & Utilization Exhibits</h4>
             </div>
             <div id="div_Container_acSum" class="d-flex justify-content-center">
               <asp:GridView ID="gvAveCostUtil_sum" CssClass="table table-responsive-sm table-bordered table-hover gridTab"  HeaderStyle-BackColor="#00AEC0" HeaderStyle-ForeColor="white" HeaderStyle-CssClass="headalign" HeaderStyle-Wrap="true" OnDataBound="gvAveCostUtil_sum_DataBound"  OnRowDataBound="gvAveCostUtil_sum_RowDataBound"  runat="server" EmptyDataText="GridViewHere" ShowHeaderWhenEmpty="true" AutoGenerateColumns="true" >
                </asp:GridView>
                 
          </div>
</div>

Now before it renders in view I have added a header manually

protected void gvAveCostUtil_sum_DataBound(object sender, EventArgs e)
        {
            GridViewRow row = new GridViewRow(0, 0, DataControlRowType.DataRow, DataControlRowState.Normal);
            TableHeaderCell cell = new TableHeaderCell();
            cell.Text = "Average Cost and Utilization Exhibits - Summary";
            cell.ColumnSpan = 7;
            row.Controls.Add(cell);
            row.BackColor = System.Drawing.Color.FromArgb(0, 174, 192);
            gvAveCostUtil_sum.HeaderRow.Parent.Controls.AddAt(0, row);
        }

In Grid View the data is bounded by

 AveCostSumDt = currentPlan.AveCostSumReport.DataSource;
 gvAveCostUtil_sum.DataSource = AveCostSumDt;
 gvAveCostUtil_sum.DataBind();

Now it wokrs fine, and the header that is manually added is shown in the view, but when I render the control inorder to export it into excel

 HtmlTextWriter htw = new HtmlTextWriter(stringwriter);
stringwriter = new StringWriter();
                    htw = new HtmlTextWriter(stringwriter);
                    divAveCostUtil.RenderControl(htw);

everything is in the htmltext but the datarow I added manually Average Cost and Utilization Exhibits - Summary is not in the htmltext. How Can I fix that?