NETGEAR AC1750 / C6300 wifi cable modem router LAN connectivity problems

I am having connective issues with Ethernet wired.  When I ping it times out a lot.  Wifi is great but internet is so slow it takes a long time to or not at all to connect.  Diganose says DNS server is not responding.  Do I change the DNS server or is their another option?  I have had cox out several times for this problem.  I did have a splitter connected it was removed today and i am still having this problem.  What can I do to correct this?