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Your ISP's DHCP is not functioning properly - ASUS router - *solved*
This post is in response to:
I had a similar issue with my Comcast/Xfinity service and wanted to record my solution to help others.
TLDR: If your internet works when the laptop/desktop is plugged directly to the cable modem but the WiFi router reports DHCP issues, clone the MAC address of the working device and apply it to the WAN port of the router.
Background: 3 months ago my internet connection started having the same problem as described in the link above - my RT-AC68U router claims "Your ISP's DHCP is not functioning properly" but I was able to connect successfully by plugging into my laptop's ethernet jack. I had a Comcast tech visit me and he replaced the cable which didn't help, finally I accepted a Xfinity hotspot device and it was provisioned and worked until today.
Today I decided to reactivate my Cisco DPC3008 modem and return the rental device. The modem connected, I was able to use the phone app to enter the MAC address into the system. I had to use the phone app to chat with a customer service rep who sent me a link to complete the activation, and after a final modem restart, I had service flowing again, with my laptop ethernet plugged directly into the Cisco modem. When I transferred the connection to my wifi router, I had the same problem appear.
I have some backup routers and tried them all in turn and every one reported the same problem - no DHCP response from Xfinity. So I tried andyross' solution and changed the router's WAN MAC address to match my laptop's ethernet port, and that was the solution. On ASUS routers the setting is found at WAN->Internet Connection->MAC Address.
I believe that Comcast has a big table of valid MAC addresses that it saves during provisioning, and the router MAC was not recognized by their system. It makes sense to prevent customers from grabbing multiple IP4 addresses on a single connection, for example by turning off NAT in the router or by plugging a switch into the cable modem and a bunch of hard-wired devices on the same switch, but it is very hard to understand and solve from this side of things.
Anyhow I hope this helps other people and saves Xfinity customer service reps a few headaches too.
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