I'm here come back to report my latest status on my China version Asus RT-AC86U. So far so good, no issue, everything are running in well condition.
This router was purchased at 24/9/2018 and received at October 2018.

Start from October 2018, this router was served me almost 10 months. So far no issue.
As long as to keep your router in best condition, you must first make sure your router is placed to an open air, clean and dry area, low room temperature condition. Make sure to prevent your router keep overheating for all day long. Also I was set my wifi smart power switch to off 1 minute for every Monday, Wednesday and Saturday at 2.00am to cycle its IP address and CCTV/alarm DDNS connections.
Currently my RT-AC86U was attached with 2pcs of 7cm fans and 1pc of 12cm ID-COOLING WF-12025 at the behind. My BTU, the grey TM brand Alcatel Lucent I-240G-T was attached 1pc of 8cm fan beside fiber optics socket area and another 9cm fan behind the fiber optics socket area. All the fans were connected with 4-pin pwm and 4-pin molex with a AC to DC universal adapter which can adjust voltage from 1.5v to 12v. So I was set to 7.5v because fans are getting noisy when set too high voltage. Another little 4cm fan was attached on the dc universal adapter to prevent it getting overheat.

So the place that I put router is sometimes getting very hot especially in the afternoon. Sometimes my switch on air-cond 24hr to cool down that area.