To use the index.html and WordPress installation in the same folder, you need to disable canonical redirects in WordPress.
To do that, follow the following steps taken from https://wordpress.org/support/topic/how-can-i-turn-off-wordpress-indexphp-redirect-to-domain-root#post-1143391 :
YOUR SCENARIO: You want to run your wordpress installation at yourdomain.com/index.php but also want to use yourdomain.com/index.html. Both of these URLs should work at the same time and there should be no redirect from index.php to index.html.
Here are the steps for newbies:
1. Open your site with an FTP program
2. navigate to /wp-content/plugins/
3. create a new folder of any name
4. create an index.php and put the PHP code from mrmist’s comments above (use everything in the code box above) and copy to your new folder
5. visit your wordpress backend and activate this new plugin
Or you can use some of the plugins, for example https://wordpress.org/plugins/ultimate-under-construction/.