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RT-AC87U getting hot, shut down. What are normal heatsink temps?

Level 12
My RT-AC87U that has been running since 2016 died unexpectedly today. I lost internet, checked the router and all LEDs were off, like it was powered off. The top of the unit was quite warm, almost hot, to the touch. I cycled the power switch several times and checked the power adapter output with a DC volt meter (19VDC). I let it cool down and tried again - nothing. I tried putting it into recovery mode by holding the Reset button in then plugging in power - nothing.

An Asus router should not just spontaneously die right (or maybe they do)? So I removed 4 screws and pulled the top off hoping to find a blown internal fuse or some sign of heat damage or a burned component. Everything look perfect and if there is a fuse, its surface mount. So despite its internals looking brand new, it was still totally dead. Then I read a post on another forum that said try removing power and slowly cycling the power switch on and off about 10 times. I don't see what that would do but I tried it, plugged it back in, turned it on, and it booted up! So maybe it over heated AND there is dust buildup in the power switch?

Now I have it sitting on a table running with the top cover off. There are 3 heat sinks attached to the main circuit board. Looking from the front, from left to right they are labelled SH3, SH2, and SH1. Heatsink SH1 is getting so hot that I can only hold my hand on it for ~1 second. Heat sinks SH2 and SH3 get warm but not hot. A quick Google search says most people consider something too hot to keep touching at ~ 50°C, so its getting at least that hot. I could attach a temp probe to it and get a real number but I don't know what the normal temperature range is for this heat sink.

The router is working (now) but I don't want to put it back into service if its going to fail soon. I don't know what is under heat sink SH1 but it is getting way hotter than the other 2 heat sinks. Digging around the internet, I have learned that many Asus routers get hot, to the point where their CPU's throttle. Some people add more heat sink mass or even install small fans. So maybe this is normal and the shutdown was a one time thing, or caused by something other than heat. Does anyone know if it is normal for heat sink SH1 to get much hotter than heat sinks SH2 and SH3 in an Asus RT-AC87U router, and what the normal temperature range is for heat sink SH1?