The Asus ax3000 is just a router, you cant just use it in place of your Comcast modem, you have to have a modem to connect to the internet, you can though use your Comcast gateway in bridge mode and connect your router and use it that way
What is the exact make and model number of that "modem" ?
@jlavaseur Yes. You are correct. I did not go into the full details of my setup as other people were seeing the same issue. I have enabled bridging on my XFINITY modem which disables the routing functionality. Connected my ASUS router to the XFINITY modem via one of the Ethernet ports. The XFINITY modem is the newer Wi-Fi 6 gateway that was released earlier this year.
When checking the internet connection via the ASUS router, the connection fails with the message "Your ISP's DHCP does not function properly."
I hope this helps.