I know this sounds crazy...

Run a second conduit to the box and attach a high volume fan to the end. Have the fan suck out of the conduit, so that the fresh air will come from where the other pipe is. And not heat up the wires.

You may have to design a shrod of sorts for the fan onto the conduit to get maximum use out of the fan.

I am assuming that you'll be installing this box and router either outside. Or an area that is prone to moisture, water.

KOT