How do I fix my header menu changes when creating a new menu?

I want to create a submenu in a page that I can access from my header menu. I created the submenu, added a line to "register_nav_menus". I want the submenu's page to have the code for theme_location corresponding to the code added to "register_nav_menus".

Now my header menu is the submenu I just created. (I updated the location in the menu admin page).

I really don't know what I can do to fix this. I am very new to WP.

Here is the code:

FUNCTIONS.PHP
// Theme Setup
function mairie_setup() {
    // Navigation resgister
    register_nav_menus(
        array(
            'header' => __('Menu principal'),
            'subnav' => __('Sous menu')
        )
    );
}
add_action('after_setup_theme', 'mairie_setup');

HEADER.PHP
...
<nav class="navbar">
    <?php
        $args = array(
        'theme_location' => 'header'
    );
    wp_nav_menu() ?>
</nav>
...
THEME FOR SUBNAV
...
<nav class="sub_navbar">
    <?php
    $args = array(
        'theme_location' => 'subnav'
    );
    wp_nav_menu() ?>
</nav>
...


I'd like my header menu to stay the header menu and not change if I create a new menu.