I was on Apex in May. The first day they give you a card in your stateroom with a suggested dining room and time "reservation". Thereafter, we booked our dinner time and venue on the Celebrity APP. It was strange, however, because the app didn't allow you to book a reservation too close to the end of your shore excursion. It didn't say that but that's what it did.
Onboard phone bookings also worked efficiently.
Mike in Ohio