Router hot

What router do you have? 

What modem do you have? 

You can always use a cheap laptop cooler to set them on to help cool them

In a general sense, most if not all major brands of router and gateways, goes through a hot box test to determine the max ambient temperature that they can safely operate in without having to throttle.

For most routers and gateways, you will see an ambient temperature range of 32F to 104F, after which, some use cases may lead to thermal issues such as throttling if the SOC is under a heavy load.

Beyond that, for thermals, the designs assume that they will not be stored in an enclosed space, or locations where the vent holes will be blocked.