Roundcube is a php webmail portal, correct? Meaning you’re running something like Apache or something to serve it. In that case, you just need to point Certify to the www-root folder on the file system so it can place the HTTP-01 challenge files. Or if you don’t have it set up like that, a virtual folder for .well-known and point Certify at the parent folder. (I think)
And, I agree that you shouldn’t give random people RDP access to a server… or even have RDP exposed to the Internet.