ASUS modem DHCP error after cable modem upgrade

I upgraded my cable modem at Xfinity's suggestion to their recommended Motorola Model MB7621. After provisioning the new modem by phone, I could not get an internet connection on a laptop wired directly to the modem. After days of calls to Xfinity support and many web searches, I accepted an Xfinity technician visit. He was able to establish an internet connection between a laptop and my new Motorola cable modem by changing some internal settings of the modem. He could not get my original ASUS wireless router to work. ASUS reported "your ISP's DHCP is not functioning properly." I was able to fix this following a post on this forum from user_113b1a, so many thanks to him!! The post was closed with no responses.

Here's how he did it and I copied that process:

1) Find the MAC address of the laptop ethernet port. You do this by getting to the command prompt (via Windows-r, enter: cmd) and using the command: ipconfig /all. You'll find the MAC address in the long list of results. Write it down. It will look like this:   [Edited: "Personal Information"]

2) Change the MAC address of the ASUS router to be the same as the laptop ethernet MAC address. Do this through the router web interface at WAN > Internet Connection > MAC address.

After I made that change, the ASUS router functioned perfectly with no DHCP error message. I hope this helps someone else.