Public IP assigned to WAN interface not working

[juicejug]

We have a situation here where we are using Comcast with 1 static IP. I can assign the WAN interface an address on the local subnet from the Comcast modem, 10.1.10.xx/24 and everything works just great. However using this address will only use our Comcast Gateway address, not our public IP. In this case I'm trying to assign our public IP x.x.x.x/30 to the WAN interface - as if in bridged mode. When I do this we no longer have Internet.

If I assign the same x.x.x.x/30 address to a computer and directly connect to the modem, the computer has internet service.

Anything I'm missing?