Sounds like that is its a hold over when the RT-N66U was going to have a active cooling system, aka a fan. The first hardware versions had it before they ditched the ROG casing. Now it just has passive heat sink, so there is nothing to control.
Had they kept it in this is what your router would have looked like on the inside and in the GUI (see link). Removed to save costs no doubt, still it might be a good idea to mount your router in such a way to maximize passive cooling by keeping it on its foot.
I would suggest a cheap laptop cooler. I personally use the one below but thats because I had it for my laptop. But there are cheaper ones too if your just going to be using it for router. I like the bigger one so I can put my router and modem on it. Then connect USB to back of router for power.