I ran into this problem and finally got past it so I thought I'd document it.
I have a tc8717 modem. The wifi is not active on it as I'm not paying for it. I had a couple of older 100mbps things conncted wired to the modem. When connecting a newer 1gb/sec thing, it couldn't get an ip address.
Solution: unplug everything, plug in only the gigabit thing to port 1, and power cycle the modem. Then the gigabit thing gets an ip address.
( Spectrum appears to have "customized" this modem, the admin app says it's not in bridge mode, yet it hands a bridge address to my old 54g router. It says there's nothing connected, when obviously there is. So don't trust the admin app. )
On the plus side, now that I can get a gigabit wired connection to this modem, my download speed is 235mbps.
I am under the impression that only one port will be active on the combo modem/router when set in bridge mode (i.e. - the wifi is turned off, and it will only function as a modem - not a router, and not a switch).
You'll need a gigabit router connected to the Spectrum supplied modem/router combo (which is set to function as a modem only) to connect multiple gigabit devices. Your gigabit router will supply wifi and be your gateway.
sometimes they will toss a seperate public IP per port... Are capable of 8 or more.
TWC res class is supposed to only send one.
It is not acting as a router, simply a bridged modem
Others will only assign a single public IP.
Correct configuration is to attach external routers wan port to port one on bridged modem, leave other 3 ports empty.