OK: This puts a whole new light on this.
Booking this late into the process almost guarantees that you will not get your choice of seating. The early and late fixed seating would have been filled long ago and open seating is what is left.
It's one of the disadvantages of last minute booking.
I would go to the maitre'd when you board and see what can be done. Sometimes there are openings due to cancellations and people changing to open seating.
If you must eat at a specific time the open seating option still allows you to do this.