I noticed you have solved several tickets simliar to what I am experiencing. This is the closest ticket to what I am seeing.
When I came home from vacation my modem was not working no matter how many times I reset it. So I bought the Arris SB8200 and had the agents activate my modem. When I directly connect to the modem it works but has a very slow internet connection compared to what I pay for (gigabit speed). My router is the asus gt ac 5300 that I have used for the last year with no issue.
I am getting a message on my router stating Your ISP's DHCP does not function properly. Could you please look into this? I would greatly appreciate the help!