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