This code may help resolve the issue for this particular situation. Put this code in yor theme’s functions.php
.
add_action('template_redirect', 'default_page');
function default_page(){
if(is_home() or is_front_page()){
exit( wp_redirect("http://path/to/your/html/file"));
}
}
Replace http://path/to/your/html/file
to exact url of html file.
I hope this helps.