Firstly, you only need to make reservations for the speciality restaurants. The main dining room (Discoveries) is open table. You just turn up and can usually go in quickly. If you have to wait the entrance is in Discoveries bar so you can sit down & they’ll call you when there’s a table.
Nowadays the reservation desk for the speciality restaurant on embarkation day seems to be in one of those restaurants, on Deck 10 aft, or sometimes just inside Windows buffet - ask at Guest Relations. It’s usually open to book as soon as people are allowed onboard