How to Add a Sub Menu Page to a Custom Post Type?

add_options_page() automatically adds it underneath settings, however add_submenu_page() gives you control as to where you want it to show up.

Try something like this:

add_submenu_page(
    'edit.php?post_type=portfolios',
    __( 'Test Settings', 'menu-test' ),
    __( 'Test Settings', 'menu-test' ),
    'manage_options',
    'testsettings',
    'mt_settings_page'
);