Unless this dusting is done on a regular basis, the dust that is now on the fan is going to be difficult to remove without any real elbow grease put into it. This rules out extension rods, brush devices that have vacuum hoses attached and all the other gadgets sold to clean these times.
You need to borrow, rent or steal a large step ladder to do this cleaning. You can hire either a handyman or window washing company to do this cleaning for you. And while they are there do any high windows in the home while they are at it. This way you get the most for your money.
You may also want them if the fixture has lights to change them to compact florescent lamps because you know as soon as they leave the bulbs will burn out.
Also a plus to have them do this is to check that the fan is still attached well. Several fans that have become wobbly do to dust on blades have shook loose from the mounting bracket and this is a good time to check to see its properly mounted.
use an extra long vacuum extension with the brush accessory at the end, even add on extensions from a similar sized vacuum if need, and stand on a sold ladder.
The Swiffer! The duster w/ the extending handle...You'll knock that fan out in no time. A friend gave it 2me when I hurt my neck & back & It was great. I hate 2 use disposable & try 2 use washable rags 4 everything but the swiffer is too good, I recommend the 360 dusters-they last longer. Better 4 u & the planet. They clean better & hold a lot of dust & dirt. The swiffer cloths however can be washed-they're great 2. Mainly 4 the floors, when u dry dust they hold the dirt in If u need 2 scrub Id use the long duster 1st then scrub then finish w/ the handle. U'll never have accumulated dust in high places again- hope this helps #LeeAnn Gavin
If I am understanding Jenn's dilemma, she said the dust is stuck on..not just loose dust? Right? Sounds like some old fashioned elbow grease is needed to rub stuck on dust off..then maintain with swiffer? I used to clean houses for many years, and found years of stuck on dust to be a challenge, once I got it off, maintenance was easy. Usually this happens in high places, like chandeliers, fans etc. You might need a ladder, scrub, then maintain.