modem firewall setting

Arris TG1682G, only network hardware, just enough to be safe because the higher the setting the slower the connection.

... and therein be the trade-off between the level of "protection" vs. bandwidth performance. At the higher security settings, each data packet is "inspected." These are called packet filters. Think of this analogous to a holiday highway drunk driver checkpoint.

My iMac is hard wired to the modem, apple 4k tv & xi5 cable box are the only devices using wifi plus a iPad that we don't use for bill paying or anything like that i think needs good security other than the iMac .Since my iMac has a firewall i didn't think cranking up the modem firewall was necessary .

For banking or any other private data security, a VPN will offer better "protection" than a firewall. Your Macs actually come with ability to be configured to use them. One recommendation here: don't be tempted to use a "free" VPN. They have to pay their bills somehow and typically do so by selling your traffic habits to other third-parties. I would suggest considering any of the following as a much better alternative: Private Internet Access, TorGuard, or IPVanish.

Getting back to firewalls. All IPv4 routers use NAT as their basic "firewall." Some routers, like your Arris, add additional dedicated firewall functionality. These firewalls, as I had mentioned earlier are packet-level firewalls that concentrate on traffic at the IP level, not at the application level.

Your Macs come with both a packet-level and application-level software firewalls. Neither are enabled by default. You access the latter firewall via System Preferences > Security & Privacy > Firewall. The packet-level one can only be accessed either by Terminal or a third-party application, like Murus.

You should turn off the firewall on the iMac if you already have one setup on your router, having both (iMac and router) is redundant and serves no purpose. Also, yes the medium setting on the router should be more than adequate. The reason your iMac has firewall setting is it can operate as a router too, it doesn't require a third party router. However most people simply use a third party router and use the default firewall settings and they are fine.

BTW I am also a Comcast data customer and have my iMac hardwired to the router/modem. I've been operating that way for years with no security issues at all.

Yup, I am assuming you are a typical user using the system for home or small business use.

Maybe ...

What is the make & model of your Comcast-provided gateway device? Is this the only piece of networking hardware in your current network configuration? What level of security are you after? That is what are you trying to keep secure that you believe a firewall will help with?