@Goldheart 

Hi, yes, it would work with Xfinity CGM4331COM gateway when Xfinity gateway is in bridge mode.

As for how to configure the DNS and port forwarding, please have a look at the following links:

• How to change DNS server settings on my Deco?
• How to set up Port Forwarding feature on the Deco?

@Goldheart 

Hi, if the Xfinity gateway is on the router mode as Deco W7200, you will have a double NAT issue which is to say, if there is a local server behind Deco, you will have to open ports twice, first on Xfinity, then on Deco.

Since your mention that port forwarding is working already, I think there is no need to bother with bridge mode anymore.

@Goldheart 

There might be two possible reasons:

1. the Xfinity might use PPPoE connection on their gateway and when setting up their gateway as a bridge mode, you might configure PPPoE broadband connection on W7200, but it is very rare for cable service.

2. the Xfinity service might lock a particular MAC address to their internet service.

--normally, you could reboot the Xfinity gateway and check the LED light later on Deco; otherwise, you might need to contact Xfinity to unbind the MAC for you.