Did you at any point try to install a phpBB mod for that purpose? I'm asking because I found this file
in the front page's source code. Unfortunately, most of the includes seem to be missing now. I've never worked with phpBB, but I've taken a look at the file and the only problem with it seems to be that it's supposed to generate XML through PHP code, but it can't do that unless it has a '.php' extension. So the idea is that you rename it 'news.php'.
But by doing so, you can't generate that code and retain the '.xml' extension you need for the feed. So one solution is to generate the code and create/update the XML file through PHP code. I've attached an example below (this isn't going to work because some of the needed include files are missing, but at least you'll get the idea by looking at the code). If none of the files were missing, all you'd need to do was open the file with Notepad and follow the instructions at the beginning. To be honest, I haven't tested it yet because I don't have a phpBB forum or the mod that was seemingly installed here, but assuming you had both, it probably would have worked fine.
That's about all I can do for now, but I'll let you know if I have any news or something you can actually use.