Not ALL the 12V Noctura fans will start up at 5V!!
For anyone facing the issue mentioned by @gattaca , and willing to spent a bit more, I can highly recommend the Noctua NFP12-Redux 1300 120mm fan with a Noctua NA-FC1 PWM Fan controller with a no-stop safety feature. I'm using an external power supply, as I've been recommended in the past not to use the routers USB as a power supply, if I recall correctly because it might cause interference. Regardless which (4-pins) 12V fan your using, this fan controller makes sure that you 12V Fan stays running and doesn't stop when lowering the voltage your feeding it, and allows you to find the perfect balance between a low sound level and acceptable temperature levels.
Mine is set just below 50% (which should be around 5V) of maximum speed (=12V/1300RPM) and with an ambient temperature of around 24 degrees Celcius my CPU temp, my current CPU temp hovers around 56~57 degrees Celsius which is more than acceptable to me. And I can't hear the fan, unless the room is completely silent and I keep my ear near my router.
Once every few weeks I set the fan controller to max for a few minutes (with the router powered off) to blow the dust out and use a rechargeable vacuum cleaner to clean the back of the router for the collected dust around the fan. See one of my previous posts for additional details and some pics if you're interested. I truly believe it has worth the extra money, as I'm now able to control the temperature of the internal components of my router to 1 or 2 degrees Celcius accuracy, which I wasn't before and I'm convinced this will prolong the lifespan of my RT-AC86U.