When you selected the custom insert just below that heading is a note:
This will show up between the 1st and 2nd post of a topic. Usually the most effective place to put ads.
This means it shows in the same column as the posts not on the side. If you hide it again in the portal layout and open the forum you will see it there.
Currently what you need to do is select the next field in the forum setting "Use a unique header/footer for this forum?
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Basically what you do is create a html table,Place the island in the table where you want it. Decide where you want the forum to appear in the table. all code before the forum goes in the header, And all code after the forum goes in the footer. Don't include anything above the table tag such at body,head tags. Of course if you use islands on the left side now it is going to add another column to the left for you unique island.
Somewhere is a good article on this but I can't seem to find it right now. I know it was in a guide for classic somewhere.
If I find it I will reply back here.
I have a couple of posts on this issue myself.
It would be a nice new feature to select forum specific island layouts now that we have unlimited islands.