No internet connection on my PCs, although the router appears to be connected.

Last week one of the PC connected to my newly bought AC68U router (with hw rev E1, and stock fw 3.0.0.4.380_3264) lost the internet connection (the network icon at the bottom right of the taskbar showed an yellow exclamation mark). I logged into the AsusWRT and there I saw that the router was connected to the internet (WAN) and starting-up the other PC I had as well internet connection.
Diagnosing the disconnected PC showed up that there was a problem with the DNS. Then I looked at the DNS...

No internet connection on my PCs, although the router appears to be connected.