How to Add an index.html Front Page to Forum
First of all, you should note that adding an index.html front page to phpBB forum is not related to phpBB system in any way. It is a completely different part which you can add to the forum. Therefore, you should have index.html created for the front page itself.
Once index file is created and ready to use, it should be uploaded to the root directory of phpBB installation. It should be the same place where forum files are located. There you also should find a file named “.htaccess” which needs to be edited. If you are using FTP client, simply download this file to the local drive and open with a file editor. Then put the following line at the very top of the file.
DirectoryIndex index.html index.php
This definition means that the browser should open file index.html and then index.php. It can be named whatever we like. For example, to set homepage as home.php, change index.html with home.php.
Finally, apply the changes by saving .htaccess file and upload it back to the server from which it was downloaded. From now whenever someone will visit http://domain.com/forum the browser will open a front page from index.html.