[masraum]

What's your exact setup, coax straight into wifi-router or coax into modem/router into wifi-router?

And just to be clear, there's no "physical" block for what amounts to electrons short of unplugging it.

I'm not sure that adding another piece of equipment would really buy you anything other than increased complexity, more possibility for maintenance and something else to break or cause problems.

I'm assuming that the coax goes into the wifi-router directly which has a built in firewall as well as the wifi on the back end. The firewall that's built into the wifi-router should be fine, and should work at the front edge of the router. If you got a firewall that went in front of the wifi-router, then the public facing IP would then have to go on the public facing side of that firewall, so it wouldn't be protected any differently other than if the firewall was just a better device.