Hey Demetrivs,
I wrote a post about this in German (I searched for a solution to make a multisite install with subdomains only accessible via https), maybe the .htaccess
snippets work for you? 🙂
https://florianbrinkmann.com/ssl-fuer-wordpress-netzwerk-mit-subdomains-erzwingen-1879/
If you do not want to touch the .htaccess
, you could try a plugin like https://de.wordpress.org/plugins/wp-force-ssl/.
Hope that helps!
Florian