Hello @Stefandj, that is really annoying when you internet restarts every couple of minutes. If you tested with a different power supply, i think it is save to say that is not the problem. 

I suggest you contact the customer department. An agent can then do a fiberline measurement to see where the problem lies in your connection. 

The new router isn't restarting anymore, but there are still multiple times a day when the internet connection drops without the router restarting. All the lights are green, but the internet drops for like 2 or 3 minutes each time