https://www.axew3.com/w3/w3all_phpbb_two_columns/
Open /wp-content/themes/your-theme/page-forum(orWhateverYouNamedIt).php
you'll find inside this explain:
Open the default page.php or template page you want insert the iframe into, to understand how to add the code, it's easy.// END MAY DO NOT MODIFY
// Start a default WordPress page
// NOTE FOR CUSTOM phpBB iframe embedded PAGES (two columns etc)
// If you need to have a custom page forum you just need:
// Copy all the above until here, before the call to get_header();
// Than copy paste all the code inside <!-- START iframe div --> and <!-- END iframe div -->
// inside whatever element the iframe need to display (for example into <div id="primary" class="content-area"> on default WP twentysixteen theme)
Here an example of the above twentysixteen page, with sidebar:
It works really good. 