need help with mobile breakpoints

I’m having difficulty changing the break points for mobile devices.

The normal main nav bar at top is not showing, even on my iPad in landscape mode. It is not sized properly by default and shows the hamburger on larger mobile device screens.
There is plenty of room for my inline menu w/o resorting to the hamburger, so I want my full menu shown, at least on tablets.

I’ve tried adjusting all the @media queries I could find and nothing sticks. This theme is determined to force defaults.

Is this hard-coded in JS, preventing customization?

I do use a child theme and its CSS file is being loaded after all other stylesheets, so it should be overwriting this stuff, but it isn’t.