Hello!
Do you guys have fans on your routers? Please show pictures!
I bought a 12V fan and connected it to the USB (5V).
Maybe instead I should have just buy a 5V fan (example: https://noctua.at/en/nf-a8-5v) and lowered the volt from OpenWRT.
Is it possible to lower the USB-voltage from OpenWRT?
Here is some picture for other cooling nerds. 
I wish you guys a healthy & happy weekend! 
While I have two routers that came with fans in stock condition (x86_64/ j1900 and ipq8074), -slightly paraphrased- why would you add a fan to a router that's been designed to work without it? Exceptions may exist for particularly harsh conditions (ambient temperatures).
Generally the hardware design really should account for realistic temperature ranges, which includes summertime in hot regions (of which south-east Asia, where most of these devices are produced and at least partially designed, would be part of), everything else would be broken hardware design[0]. Now there might be some tweaks in order (mounting vertically or maybe lifting the cover/ adding more vents), but adding active cooling to a device like this feels like defeat[1].
EDIT: looking at your temperature graph, <40 °C is nothing (presumably the wireless cores might get hotter though), my nbg6817 runs twice that hot. ARM is generally running hot (hotter than its mips predecessors), but it's also designed for that.
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[0] my nbg6817 ran particularly hot (and on average +10°C compared to the virtually identical r7800, with larger heatsinks and vents), I would call this bad thermal design on ZyXEL's part, but it nevertheless ran fine for the almost 6 years it acted as my primary router (still working, just replaced by faster gear).
[1] I took extra care that my always-on home server would be fanless - and it's been doing fine unchanged since 2013, so I wouldn't blame lack of cooling in its eventual demise.
MT6000 doesn't need any additional cooling...
I mean, if you want to do it, that's fine, but it's really not needed..
My appnet m35 is full fanless, works with stable 40-42*C , zero problems. I'm happy with it. Load with pfsense isn't high, with openwrt it was few degres lower. Not like bs n100 family mini pcs...
