I think it needs to be a hook in the template perhaps. The Uber Menu support person said:
The short answer is add it to a template – and in GeneratePress world, that means inserting it via a hook. Some of this will depend on whether you also intend for the hero image to be global, and if not, where the menu would appear on pages without it. This GeneratePress doc shows where the different hooks would place content: https://docs.generatepress.com/article/hooks-visual-guide/ – then you’d just add the ubermenu() function inside the appropriate hook callback. I’m guessing you’d want either generate_inside_container or generate_before_main_content. And then you’d use the same pattern I used to add the header-right menu, but switch out the wrapper class, theme location, hook, etc.