Page index not chaning on manual databinding to gridview

I have manually databinded my gridview , but problem is it won't show 2nd page index .... here's a chunk of code :

 <asp:CheckBox ID="SARecords" OnCheckedChanged="SARecords_OnCheckedChanged" AutoPostBack="True" CssClass="tooltips" data-original-title="View all mapped phases." data-rel="tooltip"  data-placement="top" runat="server"></asp:CheckBox>

                                          
  <asp:UpdatePanel ID="ExistingMappedUpdatePanel" UpdateMode="Conditional"  runat="server"> 
                                                <ContentTemplate> 
                                                    <asp:gridview ID="ExistingMappedGrid" ClientIDMode="Static"  OnPageIndexChanging="ExistingMappedGrid_OnPageIndexChanging"  
                                                                  PagerStyle-CssClass="custompaging"  runat="server" AutoGenerateColumns="False" 
                                                                  CssClass="table table-bordered" BackColor="White" ForeColor="Black" BorderStyle="Groove" ShowFooter="True" 
                                                                  DataKeyNames="Act_ID" >
                                                        
                                                        <Columns>
                                                            <asp:BoundField DataField="Act_ID" HeaderText="Act_ID" ReadOnly="True" SortExpression="Act_ID" Visible="False" />
                                                            <asp:BoundField DataField="do_ID" HeaderText="do_ID" SortExpression="do_ID" Visible="False" />
                                                            <asp:BoundField DataField="Act_Code" HeaderText="Activity Code" SortExpression="Act_Code" />
                                                            <asp:BoundField DataField="Act_Seq" HeaderText="Activity Sequence" SortExpression="Act_Seq" />
                                                            <asp:BoundField DataField="Phase_Name" HeaderText="Construction Phase" SortExpression="Phase_Name" />
                                                            <asp:BoundField DataField="Act_Name" HeaderText="Activity Description / Name" SortExpression="Act_Name" />
                                                      
                                                            <asp:TemplateField>
                                                                <ItemTemplate>
                                                                    <asp:LinkButton ID="RemoveRow" CssClass="btn btn-blue btn-block" OnClick="RemoveRow_OnClick" runat="server" >Remove</asp:LinkButton>
                                                                </ItemTemplate>
                                                            </asp:TemplateField>
                                                        </Columns>

                                                        <FooterStyle CssClass="panel-heading" />
                                                        <PagerStyle BackColor="PaleGoldenrod" ForeColor="DarkSlateBlue" HorizontalAlign="Center" />
                                                        <HeaderStyle  CssClass="customGridViewHeader"  ForeColor="black"/>  
                                                    </asp:gridview>
 
                                                   
 
                                                </ContentTemplate>
                                                <Triggers>
                                                    <asp:AsyncPostBackTrigger ControlID="ExistingMappedGrid"/>
                                                     <asp:AsyncPostBackTrigger ControlID="SARecords"  EventName="CheckedChanged"/>
                                                </Triggers>
                                            </asp:UpdatePanel> 

and in code-behind i did this :

    protected void SARecords_OnCheckedChanged(object sender, EventArgs e)
    {
        try
        {
            if (SARecords.Checked == false)
            {
                ExistingMappedGrid.DataSource = null;
                ExistingMappedGrid.DataBind();
                ExistingMappedUpdatePanel.Update();
            }

            else
            {
                query = "SELECT * FROM [v_ExistingMappedPhases] WHERE (([do_ID] = @do_ID))";
                var cmd = new SqlCommand(query, _connection);
                cmd.Parameters.AddWithValue("@do_ID", Session["do_ID"]);
                var dt = new DataTable();
                var da = new SqlDataAdapter(cmd);
                da.Fill(dt);
                ExistingMappedGrid.DataSource = dt;
                ExistingMappedGrid.AllowPaging = true;
                ExistingMappedGrid.PageSize = 5;
                ExistingMappedGrid.DataBind();
                ExistingMappedUpdatePanel.Update();
                _connection.Close();
            }
        }
        catch (Exception ex)
        {
        }
    }

    protected void ExistingMappedGrid_OnPageIndexChanging(object sender, GridViewPageEventArgs e)
    {
        PhasesTempGrid.PageIndex = e.NewPageIndex;
        query = "SELECT * FROM [v_ExistingMappedPhases] WHERE (([do_ID] = @do_ID))";
        var cmd = new SqlCommand(query, _connection);
        cmd.Parameters.AddWithValue("@do_ID", Session["do_ID"]);
        var dt = new DataTable();
        var da = new SqlDataAdapter(cmd);
        da.Fill(dt);
        ExistingMappedGrid.DataSource = dt;
        ExistingMappedGrid.AllowPaging = true;
        ExistingMappedGrid.PageSize = 5;
        ExistingMappedGrid.DataBind();
        ExistingMappedUpdatePanel.Update();
        _connection.Close();
        PhasesTempGrid.DataBind();
        ExistingMappedUpdatePanel.Update();
    }