Login logout below menu bar

If your theme adheres to WordPress standards the menu is generated with a call to wp_nav_menu. This function has a filter that allows you to modify the html. Like this:

add_filter ('wp_nav_menu', 'wpse312466_add_loginlink'); 
function wpse312466_add_loginlink ($nav_menu, $args) {
  $nav_menu = '<div id="loglink"><a href="https://wordpress.stackexchange.com/link/to/loginpage">Login here</a></div>' . $nav_link
  return $nav_menu
  }

Now it depends on your css skills to get that link in the place where you want it to be.

The obvious place to stick this code is in a child theme, but you could also make it into a plugin.