Margins in fixed layout epub

Hello there,

I recently bought an epub that, I think, turned out to be a fixed size epub.
It's an epub file alright, I can open it in calibre book editor and select text and stuff.

Once once my kobo tho, it behaves like a PDF: I can only do manual zooming, I can't edit the font, change size, or anything.

Unfortunately, the fixed layout include HUGE margins that makes the text much smaller than I'd like too. Is there a simple way to remove the fixed layout it, or at least a way to reduce the margins myself ?

Thanks !