I have an issue with my network. I sometimes cannot open any sites in Safari (on macOS Sierra). It takes some time (up to minutes) However, at the same time we are watching videos online (via Apple TV) through the very same sagemcom 3686 modem. Once it works again I don't have the same issue anymore, so I doubt it's a simple DNS issue. However, I didn't set up DNS on the modem, and already tried a different DNS then the standards TDC (google or openDNS).
What could it all be?