The hottest running routers are ASUS ones and they go do up to 90°C. The SoCs are designed to withstand high temps but if it's uneasy fro you, let a USB fan or similar blow at the router. I do that too in the summer as it can get pretty hot inside
ASUS' Blue Cave router has drawn a lot of attention for both its unique design and being one of the first routers to be based on an Intel / Lantiq platform. Announced last June at Computex 2017, the BC started shipping only a month or so ago. With both Intel and ASUS behind it, you'd think there would be all sorts of reviews of the product by now.