I assume that you have done all of steps that Jim has suggested above. The process is the same, however if you have the knowledge for it, you would need to edit the nginx/apache configs and .htaccess accordingly.
First off,
Make sure to change any RewriteConditions on .htaccess from /foro/ to just /. There may be some of them on the main .htaccess file you can find on the Invision Community Marketplace.
Regarding your apache/nginx configurations, I would suggest getting someone that knows how the software works and working with them as it can vary depending on your configuration.