So yes, if you want to only show the “Coming soon” page, and keep working on your WordPress site at the same time, the best option is to move your WordPress installation somewhere else and only leave the HTML5 page.
Another option would be through plugins – there are plugins that exist to force all URLs on your site to go back to this special page, unless you are logged in. But you’d have to make sure the plugin in question allows you to modify the html of the default page it shows.
Any other solution that I can think of is going to be incomplete (i.e. people could still go look at mysite.com/wp-admin or any other URL they can figure out by themselves) and would prevent you from working on all or part of your site.
Hope this helps!