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Mobile responsive page issue

I need advice regarding mobile responsive pages.
I’m struggling with a responsive design for an app that will be predominantly used on mobile devices.

I’m using a custom page with minimum width of 320px and a max width of 1280px. The main page content should be min 320px and stretch to fit a screen, up until it reaches 400px. After 400px the content is fixed width.

When I try to make this work the content is always shown as 320px and doesn’t stretch out until the screen size reaches around 1200px.

I’ve spent ages on this and I’m going around in circles. Any advice would be greatly appreciated.