ASUS Router RT-AC88U - "DHCP did not function properly"

Hi:

I am getting random/intermittent loss of network connectivity.  When I check the logs of my ASUS RT-AC88U I see the following error " WAN Connection: ISP's DHCP did not function properly"

I have to restart my cable modem (rented from Comcast - Model DPC3941T) and my Asus wifi router to restore service.  I have had the same router for over a year.

Anyone has any ideas? Can someone help. ?

Thanks