The Secondary Nav is one of the legacy sections from our older products and essentially predates responsive code. I'm afraid it would be a major undertaking to redo the code to make this fully responsive.
The only other options I can think of are:
1. use the Full-Width Sidebar where you were using the Secondary Nav and place the code for the Grid System in a text widget inside that sidebar. This is going to be quite a bit of work and will require the plugin Widget Logic so that you can apply a widget for each page where you used to have the Secondary Nav.
2. Still using the Full-Width Sidebar, you can create a custom menu in a text widget, using Widget Logic to assign the widget to the specific page.
All these are very complex solutions, so you may wish to contact one of
to see if one of them could help customize the Secondary Nav as a responsive section.