Non-wordpress subdomain on Multisite Installation

In .htaccess you have to (make sure that you set your directory right !^/../ABC/ )

RewriteEngine on
RewriteCond %{HTTP_HOST} ^(www.)?2subdomain.example.com$
RewriteCond %{REQUEST_URI} !^/../ABC/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /ABC/$1
RewriteCond %{HTTP_HOST} ^(www.)?2subdomain.example.com$
RewriteRule ^(/)?$ ABC/index.php [L] 

https://codex.wordpress.org/Giving_WordPress_Its_Own_Directory