You internet has a set amount of throughput,
probably about 1.5-3 Mb down.
To Ddos You someone basically sends large packets to your ip. They send so many of those large packets that it fills up your total throughput.
You no longer have any extra throughput for yourself as someone else is filling it all up with incoming traffic.
You can configure your router to start discarding wan pings if you like, but I don't think it will fix the problem.
If you called your cable company about the problem and they looked at it and did not see patterns of a ddos attempt on your connection you are most probably not being ddossed.