If you are a repeat cruiser on
DCL, you can make your reservation at 90 days out.
If this is your first cruise with DCL, you can make your ressies at 75 days out.
I would imagine you could have your TA do it, but I wouldn't since most likely they won't get up at 3am (depending on the time zone) to do it for you.
Palo dinners are easier to get than the brunch. If you are a first time cruiser, my guess is that brunch ressies will be gone by the time 75 days rolls around. I got to book at 90 days out, got up at 3am, couldn't get a ressies due to a glitch in the system. The brunches were gone by 8am that day.
They do not allow all reservations to be reserved on-line. A certain percent is held for the day of embarkment. But again, if you want brunch, you've got to be right on it. I had boarding card 1, and a lady with the same number about 20 people back didn't get a brunch ressie.