I think I understand this, but not entirely sure. I wanted to add the alternative I used, in case it might be helpful to others looking for this topic who want something similar but don’t know as much code as OP. It seems morriscountynj, the OP, is only using the main-navigation and a custom menu, so, I would think, the secondary-navigation is available.
When I wanted to use a different menu for mobile, I just used CSS and the customizer to have the main-navigation show up in regular view while hiding the secondary, then hid the main and showed secondary in the mobile view. I was able to do it eventhough I can’t do code this advanced.