Hello Janis,
You can absolutely make your dining reservations for restaurants at Disney World online - whether you are a Guest staying on property, off property, having booked through a travel agent or not!
Do your homework and start choosing restaurants because you can start making reservations at many Walt Disney World Resort restaurants 180 days in advance. In my experience, if you're looking at some popular locations like Cinderella's Royal Table or Le Cellier Steakhouse, you'll want to mark your calendar!
You are correct, Guests staying on property can make all of their dining reservations for the length of their stay (up to 10 days) at the 180-day mark. But you will need to call a Cast Member at 407-WDW-DINE to do so.
If you haven't taken a look already, you can learn more about the restaurants by cuisine or location by clicking
here.
Hope this helps. And good luck!