PHP not receiving form data

I'm trying to use a form with an ID (since i'll be having various forms that'll go to various tables inside the DB) to insert data to a database using php but it just won't work.

<form id="ELGEE">   
<h2>Modelo</h2><input type="text" name="mod"><br><br>
        <h2>Codigo Error</h2><input type="text" name="cod"><br><br>
            <h2>Descripcion</h2><input type="text" name="des"><br><br>
            <input type="submit" name="boton" value="Insertar" onclick="IngresarLG()">
</form>            

That's the form i'm using

function IngresarLG(){
                var mi_form= document.getElementById("ELGEE").value;
                if (mi_form.checkValidity()=true){
                                    
                    var datos_form=new FormData(mi_form);
                               
                    if (window.XMLHttpRequest) {
                        
                        xmlhttp = new XMLHttpRequest();
                    } 
                    else { 
                        
                        xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
                    }
    
                    xmlhttp.open("POST" , "insertarlg.php", true );
                    xmlhttp.send(datos_form);
                        
                }
                else{
                    alert("Datos incorrect");
                }
}

This the function i'm using that validates the form

<?php
    $modelo = $_POST['mod'];                         
    $codigo = $_POST['cod'];
    $descripcion = $_POST['des'];
    $con = mysqli_connect('localhost','root');                     

    if (!$con) {
        die('No se pudo conectar: ' . mysqli_error($con));                                                          
    }
    mysqli_select_db($con,'airec');                
    $sql="insert into lg(modelo,codigo,descripcion)";
    $sql=$sql.  " values('".$modelo."','".$codigo."','".$descripcion."')";   
    $result = mysqli_query($con,$sql);                     
    mysqli_close($con);
?>

and this is the php code that it's using.

i've been stuck here for a while and have no idea why it won't send the form data to the db. i did a similar thing a while back and that works fine but this is just making me go crazy.

source https://stackoverflow.com/questions/71562258/php-not-receiving-form-data