DoS Attack: Ascend Kill

From time to time, my Router's Log is full of entries like:

DoS Attack: Ascend Kill] from source: 17.253.34.125, port 123, Monday, December 18,2017 23:50:43
[DoS Attack: Ascend Kill] from source: 17.253.34.253, port 123, Monday, December 18,2017 23:50:43
[DoS Attack: Ascend Kill] from source: 17.253.54.125, port 123, Monday, December 18,2017 23:50:42
[DoS Attack: Ascend Kill] from source: 52.178.161.41, port 123, Monday, December 18,2017 23:34:36

...etc.

After periods with filled Logs like above, there are some days without any of these Log entries. I don't use WiFi from my Router. I use it with a FRITZ!Box connected via LAN to my Router. Router's input (Internet) is coming from a separate Modem.

Up to now, I didn't find any Community posts with '[DoS Attack: Ascend Kill]' or at least similar Log messages but with my Router's Model WNDR3800.

So I've opened here this new entry with following questions:
Is this message from a successfull DoS block of my Router?

If not, what can I do on my Router's configuration to block these DoS Attacks successfully?

Many thanks in advance for any help, info or ideas!