I spent a long time sorting this out - some with Comcast and some digging.
Our situation was that we previously had a wired LAN connected via a Router to a Comcast Modem. We upgraded to a Business Gateway with a fixed IP address - and couldn't route.
We wanted the Gateway to pass all traffic forward to our Router.
The solution was in two parts - the Comcast people were very helpful indeed and as others have said in this thread we did not actually need them to do anything.
Part One - in the Comcast SMC Business Gateway settings.
In the [Firewall] | [Port Configuration] setting
Set this to:
[x] Disable all rules and allow all inbound traffic through.
(there were lots of red herrings about the Gateway IP address of 10.1.10.1, etc. - ignore all that. The above was the only setting change that was actually needed).
Part Two - in the Router: Step 1.
Previously the router WAN had been set to be DHCP from the modem.
In the Gateway scenario that must be changed to be fixed - and must be your static IP address.
The installing tech had given us our IP Address, its Subnet Mask and the Gateway to use, and the DNS Servers.
We entered those in the Router's 'WAN IP Network Settings'.
Part Two - in the Router: Step 2.
(Browsing was fine but it would not route inbound traffic. Sheesh!)
Many hours later we discovered what the issue was!
The router has two methods of mapping inbound port traffic to the destination server (NAT) on our LAN.
1.) Port Redirection
2.) Open Ports
It turns out that if our router has the public facing IP as its WAN setting you /must/ use Open Ports. As 'simple' as that.
Previously NAT had been handled by Port Redirection, quite happily, but changing the type of connection meant we needed to change over to Open Ports. Once we had done that, then traffic flowed as expected.
I hope that helps someone!
Good luck and drink lots of tea!