If you don't have the budget for AC5300, get the RT-AC86U instead of the RT-AC68U
The RT-AC86U has faster processor, cooler running temperatures & bigger RAMs than the RT-AC68U

It has nothing to do with cooling
Read through this topic and you will see that 2 out 3 users in here with a dead AC86U does use fans on their routers but their routers still end up dead

Again, it has nothing to do with cooling, it's a manufacturing fault. Read what I said earlier, there is no way to get 100% quality on every unit

1 of my friend bought AC86u from lazada oversea seller with 3years warranty ...

after use a month the router failed to connect to unifi account ...he chat with seller ..the seller ask to send back the unit and replace with new unit ..he did sent back and received new unit after few weeks ....

few Months later the unit gave problem again ...final the seller ask to return the unit and refund the money .

..the ASUS AC86u failure rate is high ....after get know from here few units having problem ...

ax88 or ac5300 less ppl use ..so less failure rate ....haha