Hi Celestina!
Congratulations on your fall visit to Walt Disney World! I'm visiting in September as well and had the same issue as you regarding
Fantasmic! Dining Packages online.
I would highly recommend that you give Disney Dining a call, right away today at (407) WDW-DINE (939-3469) and make your Fantasmic Dining reservation directly with a Cast Member there. They can access the schedules that far in advance on their end and you should be able to secure something that way!
T-REX is a very popular dining location, both because it's newer to Walt Disney World and brand new to the Disney Dining Plan this year. It may be that the restaurant truly is already fully reserved by other vacationers like yourself, arriving in September. That being said, plans change constantly, so check back online often and you may get lucky with a cancellation that suits your vacation itinerary!
When you call Disney Dining, it also wouldn't hurt to have the Cast Member check for ALL availability for T-REX during your stay as you may have better luck that way with a highly coveted reservation such as that one!
Wishing you a very Magical time on your Walt Disney World vacation Celestina!
Kirsten