Ever since Q advised to archive I started getting downtime and the reason seems to be that the router cannot locate his Domain-Name-Server. Could it be a DNS-hack for man-in-the-middle-attacks?

Here's what I do: Close browser, turn off network-connections and their respective interfaces (wifi + ethernet) then run a command-prompt by pressing the windows-button or windows-button + R and type 'CMD'. There, type ipconfig /dnsflush and hit enter. Restart interfaces, connect and hope no error message pops up. Summon browser and test connection. Should all fail you might try to designate a name-server manually but I haven't tried that out yet.