The cast member you spoke with is mistaken.
Just this morning, I used the 90 + 10 rule myself to make online dining reservations for my entire vacation. If you are staying at the Pop Century, you can certainly do this too.
To make online reservations for your whole trip, you'll need to let Disney know that you are staying on site. On the reservation pages for the restaurants you'll see at the top of the page:
Log In or Register
If you log in or register before you make your reservations, you can retrieve and view them later online.
Retrieve Your Resort Reservation
Do you have a Resort reservation for the same days for which you're making dining reservations?
If so, please retrieve your reservation.
You should log into your Disney account AND input your hotel reservation confirmation number in the appointed areas. Once you do this, the online reservation calendar will expand to include the entirety of your vacation.
Good luck and have a wonderful visit to WDW.