Use the following shortcode [show_theme_switch_link] in templates to show the theme switch link.
Example: <?php echo do_shortcode('[show_theme_switch_link]'); ?>
You can also add Switch Mobile Theme link to your Menus from Custom Links section under Appearance > Menus.
Example:
Url : /?=desktop (For Mobile Theme)
Url : /?=mobile (For Desktop Theme)
Label : As you wish :)