@tERROR 

Hi,

I don't know why the Archer C1200 becomes stuck, but the "led-controller" WAN0 and WAN1 entries in the logs do not refer to different WANs. Led-controller WAN0 and WAN1 refer purely to the two LEDs of the router's "Internet" indicator light. WAN0 should be the orange LED (port is physically connected, but there is no Internet) and WAN1 the green LED (port is physically connected and Internet is available). Basically the log you posted tells you at what time the "Internet" indicator switched to "orange" or "green" (or no light at all).

It was explained like this by a TP-Link representative some time ago.

  @woozle 

Yes, I read that.
I'm trying to figure out whether the connection drops due to packet loss (ISP) or because the router has stuck.

Actually, I'm not sure if the entire device has stuck; it might just be the web interface that's unresponsive. The background of the page loads, but not the login form. At the moment the connection is restored, the router starts responding.

  @tERROR 

Are you browsing to the web interface via tplinkwifi.net or via the router's actual IP address? In case it's the former, then check if using the IP address makes any difference. (e.g.  http://192.168.0.1 ) 

Also, when the C1200 gets stuck, test whether it becomes resposive immediately if you disconnect the Ethernet cable from its (blue) WAN port.
 

  @woozle 

I am using the IP address. Ok I will try the cable. That's a good idea to locate if its internal device  problem.

  @tERROR 

After conducting some tests, I found the following:

• The colors of the router's indicators do not change during the problem. They remain green. The router is so stuck that when I unplug the cable for the external network, the indicator remains green.
• It is not caused by the external network.
• The internal network continues to function; only the router freezes.
• It is an issue with a device on the internal network. I isolated the problem to a extender (TL-WA850RE) that I use to share my network with my neighbor while they're in the country. I suspect it's some kind of iOS service or a virus — I'm not that familiar with Apple devices. Another possibility is some type of attack on the extender. 
• The fact is, however, that when I turn off this extender, both the router and the internet come back to life.