I have understand the issue, that you wish to create a custom page as default for the server to show and from that page put a link to your forum.
Instead of deleting the index.php of punbb you may used the .htaccess directives :
1.Create your custom page and upload it to the server, put a name like portal.html.
2.Create a file called .htaccess and add the following in it
3.Close, save and upload it to the root directory of your account.