This is for a multisite that has 3 levels of organization: main site - regions (subsites) and locations (sub-subsites). So you would be able to visit and administrate these urls:

domain --> main site domain/region --> sub site domain/region/location --> sub-sub site

Both front and back end are getting loading errors in the Inspect Console because the path to the files in "wp-includes" and "wp-admin" are wrong.

I have sunrise.php activated.

These are the lines I added to wp-config.php:

/* Multisite */
define( 'WP_ALLOW_MULTISITE', true );
define('MULTISITE', true);
define('SUBDOMAIN_INSTALL', false);
define('DOMAIN_CURRENT_SITE', 'MY-DOMAIN'); // <- my real domain there
define('PATH_CURRENT_SITE', '/');
define('SITE_ID_CURRENT_SITE', 1);
define('BLOG_ID_CURRENT_SITE', 1);

define("WP_CONTENT_URL", "/wp-content");

define('ADMIN_COOKIE_PATH', '/');
define('COOKIE_DOMAIN', '');
define('COOKIEPATH', '');
define('SITECOOKIEPATH', '');

// Activate sunrise script
  define('SUNRISE', TRUE);
// define('WP_ALLOW_REPAIR', true);

This is my .htaccess file:

# BEGIN WordPress
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# uploaded files
RewriteRule ^([_0-9a-zA-Z-]+/)?files/(.+) wp-includes/ms-files.php?file=$2 [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]

# RewriteRule  ^[_0-9a-zA-Z-]+/(wp-(content|admin|includes).*) $1 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)+(wp-(content|admin|includes).*) $2 [L]

RewriteRule ^[_0-9a-zA-Z-]+/(.*\.php)$ $1 [L]
# RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]

RewriteRule . index.php [L]
# END WordPress

This is my sunrise.php file