TR:DR: Just change the router to a better router hardware, you will not regret it.
Hi, here is the status of Archer C1200 v1 after being used for half of year, keep in mind that it is not having heavy users that p2p downloading everyday, but sometimes.
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For the first 3 month, it will reboot once per month.
After update to firmware that fixed the Google multicast crash issues(should be after 1 more month), it will reboot once per weeks.
Starting since this month(as everyone having school break, only me use the UniFi), it has been become reboot once per day, before I tried to fix it by updating the firmware to latest version Build 20180403.
As it is reboot everyday, I observed the system log sometimes will be wiped out after the crash, and sometimes it will failed to reboot, which caused it to be reboot twice.(at least 5 mins)
It is very similar like some old Android Samsung phone that will crash and reboot itself while also causing data corrupted, unless you set the Performance governor that forced highest frequency.
So I guess it is hardware degradation, and unlucky that I normally don't p2p downloading, so it will underutilized and undervolted(as NAT Boost is enabled), so after running for a long time, it just crash.
Or it can be the PSU is falling to having a constant supply as someone using the same extension plug for kettle, which causing unstable pulse for the PSU and damaged it permanent.
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Anyways, I had just reset the router, setup everything from scratch, and will continue observe this crappy router, before TM start providing new router or having the money to exchange it with a better one.
Here is the latest reboot time with latest firmware version: 1.0.0 Build 20180403 rel.22124 (EU) (TM UniFi).
(count from Reboot, not boot time, so it can simulate when the router crash happen.

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02:09.8 Router Finish Rebooted/Web GUI available/LAN connection re-active (WebUI still slow on loading and 100% CPU usage)
02:54.7 Internet available (still 100% CPU for 30-60 second)
I only tested with UPnP enabled(disabled by default), File sharing disabled(enabled by default and the start activity will be logged), Tr069 disabled(enabled by default).
It is very boring when it crash at night, as the TM ISP will disallow you to login freely like 5-15 mins, before the system discover your old PPPoE connection has been disconnected/logout.