Currently Menu Plus only works with the primary menu.
However, you can just style your secondary navigation like the primary nav, and the primary nav like the secondary nav. They both have the same options, to they can be changed around π
In fact there was a plugin called “Mobile.Nav” which was doing this. I wasn’t aware of it as i took over the site. It was there because the previous theme was not as good as GeneratePress π
So I just needed to disable Mobile.Nav and it all worked as expected.