No access to settings [multisite]

Tried the solution here, like posted above: https://blog.nintechnet.com/restricting-access-to-ninjafirewall-wp-edition-settings/

NinjaFirewall (WP edition) troubleshooter
HTTP server	:	Apache/2.4.10 (Debian)
PHP version	:	7.4.7
PHP SAPI	:	FPM-FCGI
 	 	 
auto_prepend_file	:	/xxx/public_html/wp-content/nfwlog/ninjafirewall.php
Loader's path to firewall	:	/xxx/public_html/wp-content/plugins/ninjafirewall/lib/firewall.php
wp-config.php	:	found in /xxx/public_html/wp-config.php
NinjaFirewall detection	:	NinjaFirewall WP Edition is loaded (Full WAF mode)
 	 	 
Loaded INI file	:	/etc/php/7.4/fpm/php.ini
user_ini.filename	:	.user.ini
user_ini.cache_ttl	:	300 seconds
User PHP INI	:	.user.ini found -
 	 	 
DOCUMENT_ROOT	:	/xxx/public_html/
ABSPATH	:	/xxx/public_html/
WordPress version	:	5.6.1
 	 	Multisite installation detected
WP_CONTENT_DIR	:	/xxx/public_html/wp-content
Plugins directory	:	/xxx/public_html/wp-content/plugins
User Role	:	Administrator
User Capabilities	:	manage_options: OK - unfiltered_html: OK
Log dir permissions	:	/xxx/public_html/wp-content/nfwlog dir is writable
Cache dir permissions	:	/xxx/public_html/wp-content/nfwlog/cache dir is writable
NinjaFirewall (WP edition) troubleshooter v1.9.4

Are you accessing https://www.otherdomain.com/wp-admin/network/admin.php?page=NinjaFirewall?
If you do, is there any error returned?

Do you see NinjaFirewall’s icon in the admin tool bar. By default it is visible in a multisite installation and, when moving the mouse over, it will show a tooltip with a link to its settings page.

yes, I see the icon and the plugin is definitely active and working. The error message is “Sorry, you are not authorized to access this page.” (wp-die / Error 403)

When I look in the plugin overview page https://www.newdomain.com/wp-admin/network/plugins.php, instead of the settings there is the gray text “Access restricted”, see https://imgur.com/a/wdQ0EK0

This restriction occurs when you either don’t have the manage_network capabilities or if this is not the main site, i.e., the is_main_site() function returns false.
Copy the following code, save it to the file, e.g., “whatever.php”, upload it to your site inside the main root folder, and go to https://your-site/whatever.php:

<?php
require 'wp-config.php';
if ( current_user_can('manage_network') ) {
	echo "OK: You have the manage_network capability.</p>";
} else {
	echo "ERROR: You don't have the manage_network capability.</p>";
}
if ( is_main_site() ) {
	echo "OK: This is the main site.</p>";
} else {
	echo "ERROR: This is not the main site.</p>";
}

OK: You have the manage_network capability.
ERROR: This is not the main site.

OK, got the point, I have to change the wp-config.

8 is the Blog-ID of “otherdomain.com” what is my new mainsite.

define(‘BLOG_ID_CURRENT_SITE’, 8);

following the hints in here

Thanks & regards, Andrea