Have you tried physically creating a file at
/onlineshop/product/view/5/14.html with the replacement HTML code? Works when I tested it just now.
If a physical file exists at that request Joomla shouldn't come in to the picture. The server should serve that physical file instead. This is assuming you're running on an Apache server and your .htaccess is set up correctly (which it looks like it is)
Why this should work:
When someone requests
dormo-novo.at/onlineshop/product/view/5/14.html from your server the server will discover that
/onlineshop/product/view/5/14.html is not a real folder or file on your system. However, your .htaccess file it tells the server, "hey, if someone requests a folder or file that doesn't exist, send that request through Joomla so that Joomla can attempt to handle it" (That's how Joomla SEF URLs work). But, if the folder/file does exist then the server will serve that folder/file instead of redirecting the request through Joomla.