Motorola MB7621 and Amplifi HD router

I am new to xfinity service.

I have been using my Ubiquiti Amplifi HD router for several years without much issue.

I am using a brand new Motorola MB7621 modem.

My service was activated a few hours ago and via ethernet cable between modem and computer, the itnernet is working just fine.

I reset my AmplifiHD to factory as I was having a problem with the router to obtain an IP address from the modem.

The same problem is still persisting where I am not getting an IP address issued to my router.

My router can be setup for DHCP, PPOE, or Static. I believe xfinity is DHCP but I would like the router to get an IP address first.

Anyone have firsthand experience with Amplifi HD systems and this MB7621 modem on xfinity? I believe it should be fairly "plug and play" and not be this difficult.

There is virtually no "settings" in the modem so putting it in "bridge mode" isn't a solution as the device is strictly a modem.