WDR3600 hangs after some time

Hello I use OpenWrt for several years now. I have one WDR3600 (let's call it R) as my main router. 4 WR841N as Dumb APs and a 16 port unmanaged gigabit port switch are connected to R.

However since a month all of R LEDs will start blinking in the same frequency until I restart It. Clients trying to use the network(Wireless or Cable) wont get an ip address and become unable to use the network, but they will detect that there Is a network connection. Clients which already had an ip address wont be able to use the network aswell. There are no errors in the system and kernel logs so I don't know where to start. Are there other logs I should check? This happens like 2-3 times a week.

However there were two changes that happened to the network:

  1. I installed a new WR841(my 4th) with a new cable with a new network socket. (Could wrong wiring cause this issue, usually all access point work with good speed, pings with no packet loss)
  2. The power adapter for my cable modem broke and I replaced It

I tried to use my old WR1043ND but It results in the same problem.
I use OpenWrt 18.06, sqm(with all default), 7 static routes, stubby(as dns resolver), ssh is enabled via lan, one firewall rule to block traffic(lan -> wan) from one mac address.

Your symptoms are a bit vague, but from personal experience the tl-wdr3600 is stable with current OpenWrt - which would lead me to looking at the hardware side first. These routers have reached an age where PSUs might be on their way out, or condensators drying up - which means I'd start with trying a different PSU first.

You mean two completely different devices both hangs in the same manner?

Sorry for the late answer, yes another device same way It hangs.

If two different devices experience the same issue, I would bet this is related to either some package you install, or perhaps both run out of memory.

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Ok I think I found the error source. Thanks for this hint. I was using Stubby as Dns proxy for Dnsmasq. Without Stubby the router runs for 2 weeks now without any crash. The problem is when the router crashed there weren't any logs but It seemes like Dnsmasq was not running so I guess Dnsmasq runs out of memory and crashes. So what would be the best way to figure out the problem because I would prefer to use Stubby.