Now the rt-ac86u had some quite specific temperature problems in the latest firmware series that pushed temps up 15 or so degrees. I don't believe that the same issues were present in the rc-ac68u though, but there might have still been a minor increase recently.
Based on that, I would say that any software fixes are out.
Firstly, I would move the router off the hot PC. That will be doing it no favours at all.
You can probably drop the temperature 10 degrees by just moving it to a nearby shelf, benchtop or desk. Or install a new shelf on the wall if one isn't readily available.
Secondly, you could check to make sure the cooling vents aren't clogged with dust.
And finally, I would look into a fan. A 5V USB powered fan would work, but you want to make sure you get one that won't be too noisy (a cheap one with a sleeve or ball bearing fan might be too loud).
Also keep in mind that forcing air throught the router will attract more dust which will need to be cleaned out periodically.