I guess it depends on the cruise. We had both dining rooms open, one for fixed and one for open seating. I chose fixed - same servers, same table every night makes for a much more pleasant dining experience. When i need flexibility I dine elsewhere.
when I sailed I thought it was a bit silly to have both dining rooms open, it just didn’t make logistical sense for the passenger levels. It seemed more people were opting for other dining rather than the MDR.
Anyway you can call guest services and request fixed dining, early or late. If you prefer open dining you can make reservations from the app. Finally you can simply talk to the maitre d’ in person