If the app is working, then the easiest thing to do is to contact dining services or guest services through the app. If the app isn't working, then just go to guest services. There's usually not a line and they don't seem to care if you have an appointment.
In my experience, the app pretty much never works for the guest services queue. I tried it multiple times on my last two cruises. Once, I was #2 in the queue and then someone from GS sent a message to help me when it was my turn. But, the WiFi kept cutting out and even though I sent my replies, I never heard back from her again. Every other time, I would join the queue and it would slowly count down until I was #1 in the queue, but then it would bump me back up to some other number and I'd never get to talk to anyone. That happened multiple times. My mom tried it, too, and her queue position just never changed.
Thankfully, guest services in person was great!