so i have seen a lot of u guys use the cooling fan separately.. i watch a few youtube videos and i made this so that i have a better flexibility to place the modem anywhere by merging the cooling fan with router. thought i share it with everyone so here it goes.
first u have to check the router's power adapter at the output part. so what i have here is a 12v output..
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i then bought a 12v dc standard pc cooling fan then i pull of the wire other than black and red. the one i bought had an extra yellow colored wire.. its not necessary so i removed it.
as the fan will spin at high rpm, we need a stable place to put it. i kinda ran out of ideas so i just stick it to the modem's heatsink

u can see it at the picture below
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so after a few tries with the long wire (trying to fit it in the case) i soldered it at the power port legs like in the picture.. u might wanna try touching those with the wire and see whether the fan spins or not.
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voila, everything quite done now.. honestly i hate this part. i drew a circle to fit the fan with the case. had to remove the centre because of the fan's position. then is just make small holes around the circle just like blacktubi's mod. i carefully then knock the part out and had kinda like jagged circle (cant affortd a dremel

) lastly i filed all the jagged corners until the fan fit.
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anyway dust is quite a problem now

i had to clean it once in a while... and if you feel that the fan drain power until the modem's perfomance drop, u can opt to buy 12v 2a power adapter if u like. mine quite ok.. no drop whatsoever. beyond these things that i mentioned everything works perfectly

Good job and thank you very much for sharing.