You can see the higher fireworks from various places in WDW but since Illuminations has quite a bit of the show that takes place on the lake you really need to be IN Epcot to see it.
If you don't want to enter the park then your next best viewing area would probably be just outside the International Gateway although you still will not see the lower stuff that happens on the lake.
you can see most of the fireworks from parts of the boardwalk, though you can't hear the music. When we've stayed at the Yacht Club and the Boardwalk Inn we've been lucky with our rooms and watched them from balconies.
We stayed in the 50s building last year, 4th floor facing the lake and we could see Illuminations from the room. It was not a great view but you could see it.
Heck, if you go to see Fantasmic, just stay in your seats while the crowds file out, and you can see the fireworks from there ... well, the really high ones, anyway.
Really, to "see" Illuminations, you have to be in Epcot, IMO.
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