AFAIK, you'll have to wait for a version 19 viewer.
As per Bob, the v19 viewer has not been approved by Apple yet. You can't view a 2048 folio in the v18 viewer.
You can build your own v19 viewer, but unfortunately, you can't do that with a free account. (So you're stuck waiting)
You can now - the v19 viewer went public today.