Hi,

I suggest the following solution: 

@media screen and (max-width: 620px){
           #wpforo #wpforo-wrap .wpforo-main {display:flex; flex-direction:column-reverse;}
           aside#wpforo_forums-2, aside#wpforo_recent_topics-2, aside#wpforo_recent_posts-2 {display:none;}
}

Please note that this code removes the rest of the widgets on mobile devices; otherwise, the entire sidebar will be above the forum section.