404 if default url contains an underscore

I’m new to GRAV, but i really like it. I want to use it to replace a REALLY old plain html website but keeping the “simplicity” of writing content.
But i’m currently stucked on a problem with the routing. I try to replace the default route of a page with the original page url of the plain html page. This site should have some underscores in name like “some_really_fancy_site.html” while the default route was “some-really-fancy-site”. But it seems that the underscores (i’ve tested it) are a problem so that open the page with the new route run into a 404 page not found.
Is there anything i can do?