Doubt - Dependent Combobox with Classic ASP

I have two tables in SQL: A group that has two columns: id and group

A product that has three columns: id, group, and product

In the Products table, the "group" column is equivalent to the "id" column of the groups table.

I'm trying to make an inclusion form into a third table of entries (columns: group, product, and entry).

In this form, there is a combobox in which the user will select the Group and, when selecting the Group, should appear in a second combobox only the Products related to it. (similar to the state / city combobox).

The webpage is connecting right into the database, but I am not able to make the combobox dependent.

I built it as follows:

Combo of two groups:

<%
SET RS_grupo = conexao.execute("SELECT * FROM [dbo].[nova_pd_grupos] ORDER BY grupo")

grupo1= RS_grupo("grupo")
idgrupo=RS_grupo("id")
grupo=request.querystring("grupo1")

%>

<select class="custom-select custom-select-sm" name="idgrupo" onChange="location.href('index.asp?idgrupo='+formulario.idgrupo.options[formulario.idgrupo.selectedIndex].value)">
                                <% while not RS_grupo.eof%>
                                <% if grupo <>"" then
                                   if grupo = grupo1 then
                                   response.write "<option value="&idgrupo&"&grupo1="&grupo1&" selected>"&grupo1&"</option>"
                                   elseif grupo <> grupo1 then
                                   response.write "<option value="&idgrupo&"&grupo1="&grupo1&">"&grupo1&"</option>"
                                   end if
                                   else 
                                   response.write "<option value="&idgrupo&"&grupo1="&grupo1&">"&grupo1&"</option>"
                                   end if

                                   RS_grupo.movenext
                                   wend
                                   RS_grupo.close


                                %>


                            </select>

Combo of products:

<% if request.querystring("idgrupo") <> "" then %>
                                <select class="custom-select custom-select-sm" name="produto">
<%  SET RS_produto = conexao.execute("SELECT * FROM [dbo].[nova_pd_produtos] where grupo = "&request.querystring("idgrupo")&" order by produto asc;")
                                while not RS_produto.eof

                                   grupo = request.querystring("grupo")
                                   produto1 = RS_produto("produto")
                                %>
                                <option  value="<%=produto1%>,<%=grupo%>">
                                    <%=produto1%>
                                </option>

                                     <%
                                        RS_produto.movenext
                                        wend
                                        RS_produto.close
                                        %>
                                </select>
                                <% end if %>

However, 13 options appear in the group list, all with the name of the first option. And in the list of products nothing appears.

Can anyone help me? Classic ASP (vbscript).

Thank you!