Elementor submits its form via ajax, so you can’t just do a $_POST grab on a page submit. You need to use its hooks.

Form submit complete (with optional redirect)

//**********************************************
//**********************************************
//********** ELEMENTOR FORM SUBMITTED **********
//**********************************************
//**********************************************
add_action( 'elementor_pro/forms/new_record', function( $record, $handler )
{
  $form_name = $record->get_form_settings( 'form_name' );   //<<<Set in form widget > Content > Form fields > Form name


  if ($form_name == 'my_form_name')
  {
    //----- HANDLE my_form_name FORM SUBMISSION -----

    //CREATE AN ARRAY OF ALL THE SUBMITED FORM FIELDS
    $RawFields = $record->get( 'fields' );
    $FormFields = [];
    foreach ( $RawFields as $Id => $Field )
        $FormFields[ $Id ] = $Field['value'];

    $my_form_field_id = '';
    if (isset($FormFields['my_form_field_id']))   //<<<Set in form widget > Content > Form fields > Select the field > Advanced > ID
      $my_form_field_id = $FormFields['my_form_field_id'];


    //Cause a re-direct
    //(This will work on a form with nothing set in 'Actions after submit')
    $redirect_url = '/my_page2';
    $redirect_to = $record->replace_setting_shortcodes( $redirect_url );
    $handler->add_response_data( 'redirect_url', $redirect_to );        //Set redirect action to handler

  }

}, 10, 2);