No Internet Access

A few days ago I bought internet-only service from Cox in Tempe, AZ. Cox added the MAC address, etc. of my modem (a Motorola SB6121) that was working fine with Comcast internet service at my old apartment in Nashville, TN.

But now at my new location with Cox I can't get online. 

I can access my modem via it's web interface and do a reset. But as soon as it's connected to Cox and Cox's files load I can no longer access the modem's web interface. And, of course, I have no internet access.

If I hook up the modem directly to my laptop via Ethernet and run Windows 10 Network Diagnostics I get " 'Ethernet 2' doesn't have valid IP configuration." Disabling the network and then re-enabling it does not fix the problem. I still get "invalid IP configuration." 

I have "Obtain Address Automatically" checked in both IPv4 and IPv6 properties.

The lights on the front of the modem all light up solid, except for the bottom light (the network light) which blinks.

It would seem like the problem is in the connection to the modem once the Cox files load. I'm puzzled. If I use a wireless router instead of Ethernet I get the same situation -- can't access the modem and have no internet connection. Neither my router or direct Ethernet can communicate with the modem once the Cox files load. 

Any help?