I have used the Buffalo WHR-G54S and Linksys WRT54G in some pretty extreme heat. I have had the Linksys running in an Attic for two summers now and the Buffalo for one summer. The attics reach approximately 120 °F (49 °C); well above their rated temperature.
I also have the buffalo in Rootenna boxes since last fall. Again no problems. In the Rootenna, I take them out of their plastic shell for better air flow. I haven't measured the temps, but last winter we saw some nights below 8 °F (-13 °C).
These consumer routers seem to do better will with heat and cold than their ratings suggest. Probably because of their low power consumption.
I'll be putting some Meraki Minis to the test this summer in Rootennas.