DHCP not assigning private IP addresses

I have a new C3000 Cable Modem/Router connecting to a new Mediacom internet connection.  I verified I could get on the internet with one wireless device before installer left, but afterward discovered that I could not connect other devices to the internet.  Here is what I've been able to determine:

The device that can connect to the internet is obtaining a public IP address from Mediacom (50.x.x.x) rather than a private IP.  I am able to ping the default gateway in the same subnet but cannot log into it (http or telnet).  So I do not think the gateway is my local C3000; I think it must be a Mediacom router.  I connected a laptop to the ethernet port of the C3000, but still could not log into it, because I could not get an IP address.  It just had the default Windows IP of 169.x.x.x.  The original device connected was an iPhone.  By having the iPhone forget the network, I was then able to pull an IP to the laptop; again a public IP address; probably the same one the iPhone had, but I am not certain.  The laptop could then get on the internet, but I still cannot log into the local router to check any configuration.

After reboots with no change, I performed a factory reset on the C3000.  Still no change.  Is this a bridge mode issue?  It's like DHCP is not working properly since it is not assigning a private IP address in the 192.168.0.x range.  I tried to hard set the laptop to 192.168.0.5 with a gateway of 0.1, but still could not ping or connect to the C3000.  I'm not sure how I can fix it if I can't log into the router.  Is there a setting with Mediacom that somehow affects the router's configuration so that it is not truly going to the default settings?

Thank you in advance.