[URGENT] Got 404 error with Grav in subfolder and Wordpress in root

I had installed a Wordpress blog in the webroot of my website. Then I have installed Grav + Admin in a subfolder (e.g. http://website.com/ is Wordpress, http://website.com/sub/ is Grav).
When I visit Grav’s URL I get the Wordpress’s site 404 error page.
How can I fix?
Thanks in advance!

It’s clear to me this won’t work without some tweaking. Before I attempt to explain, why have you made this setup?

[Edit: and why is this urgent?]