large issue with iframes no matter which one

Hi,
I want to include an mchat iframe on my WordPress page. I have a serious problem with creating iframes. They do appear (for example the custom iframe or the mchat iframe), but in terms of height they never scale the right way. I followed the exact steps of https://www.axew3.com/w3/2020/01/phpbb-wordpress-template-integration-iframe-v5/ and also worked with https://www.axew3.com/w3/forum/?coding=dmlld3RvcGljLnBocD9mPTImdD05MTE= and https://www.axew3.com/w3/2019/12/w3allcustomiframe-shortcode-how-to/

After all of this, the iframe still isn’t scaling the right way :/
I also tried setting the custom CSS editing option in w3all admin config and edit the custom_js_css.php for myself and also still didn’t worked. Automatically recompiling is also activated at PhpBB and all the necessary changes also with copying the
iframeResizer.contentWindow.min.js and iframeResizer.contentWindow.map where made. overall_footer code also has been coped right above the </body> also with changing the domain paths.

WordPress domain: https://theheavenlydownload.com/
PhpBB directory: https://theheavenlydownload.com/forum/
I just want to include mchat or the custom mchat page https://theheavenlydownload.com/forum/app.php/mchat as custom iframe

The page I need help with: [log in to see the link]