Hello, because the file
/wp-content/plugins/wp-w3all-phpbb-integration/addons/page-forum.php
that's copied into your active template folder as
page-(whateverYouNamedIt).php
when you click to create the forum template page, it is copied as is, without sidebars.
SO you have 2 solutions:
or you edit manually the created file into the active template folder, if you know how, OR, now that you created it, and leaving it as it is there, i suggest to use
The (awesome) iframe template integration using shortcode
See inline instructions into
Wp admin -> Settings -> WP_w3all -> WP_w3all Preferences -> The (awesome) iframe template integration using shortcode
Create a page into WP admin and add the shortcode following instructions
and if any trouble just post!
see simple example here:
https://www.axew3.com/w3/board/
see inline option instructions and
https://www.axew3.com/w3/2022/02/the-aw ... shortcode/