Before investing a lot into cooling a router, if there is no security tag, then take it apart, or stick a thermal probe through a vent hole, then test the temperature of the CPU and wifi radios. (though that fan mod is the safest route to avoiding any possible warranty issue)
You can then look up the model numbers of the CPU and wifi radios and find their max operating temperatures.
For many router components, when run within spec, anything under the max temperature will offer the same stability as all other components.
Even on a hot day, many routers, even high end ones, will actually run well below the overheating temperature.
if temperatures get too hot, then consider buying some thermal tape, then attaching a small heatsink to it.
for example, on one of my routers, I added a few heatsinks (also modded it to add some external antennas), and it almost cut the temperatures in half. The components do not use much power, but have a decent amount of thermal density, and can get very hot, but once a little bit of thermal mass is added, they will cool down significantly.
I had to do this for the summer, only for this router as it got hot enough to cause throttling. My other routers such as the R7000, have no thermal issues, even when ambient temperatures are really high.
The only time when active cooling really helps, is when you have a a router with heatsinks and is still very hot, for example, my crappy verizon fios actiontec router (had to turn it into a moca to ethernet bridge), and use another router as the main. (my actiontec crap gets over 90C )
Depending on the construction, many companies will be unable to tell if a router has been opened. if you want a 100% reversible mod, then you can stick a little bit of arctic silver 5 thermal compound on it (if the router will be placed horizontally, then the heatsink will remain in place, and will not fall off unless it is held at an angle for a while).