The 404 at your installation root is to discourage bots from crawling your FunnelFlux installation.
Your dashboard is located at yourdomain.tld/admin/
You can change the default behavior by editing the file
In this file, you will see a line such as the one below:
const DEFAULT_URL = '';
If you set a URL instead of the empty value, your install root will redirect to that URL instead of showing the 404.
const DEFAULT_URL = 'http://yourdomain.tld/blog';
...would redirect to your blog.