Solved. The 23 theme I *was* using generated the menu that I couldn't change, with no obvious way to change it. This theme's default settings did not display my wpForo navigation menu at all. So to me, a newbie WP user, it looked like the changes I made to my menu were not taking effect.

Figured this out when I switched to the much more easy-to-use Astra theme, which does display my wpForo menu.