Note that by default, the AP is a bridge (imagine it as a wired switch, but for wireless), and Comcast will not like giving you multiple public IP's in this fashion. You'll need to configure your wireless SSID to a "local subnet" configuration, which allows the R700 to take on the role of a NAT route and hand out private IP's behind your NAT.
Also, remember that Comcast modem firmware generally "marries" to a particular MAC address once per bootup, so you need to reboot your modem when changing what the modem is connected to.
But personally, I don't recommend this direct setup. I would strongly recommend inserting a wired NAT router in between. While the R700 can do NAT, it's not the world's most feature-filled NAT router,, and sooner or later you'll want port forwarding or UPnP or static leases / static DNS, and then realize you're out of options.
I would strongly recommend either buying a wired router (e.g. turn the wireless portion off), or since you're a home user, DD-WRT works great too as a wired router. Then setting up the R700 behind it will be simple.
John's reply gives you basic setup, but I'm not sure if you have a network, or just a Comcast modem.
If you have an existing network with Internet access thru an ISP, you can add the R700 to your network with a static IP/netmask/def-gw on default VLAN 1, assigned under Configure::Internet.
Test your access from the R700 from Administration::Diagnostics, where you can Ping local and remote targets.
The R700 is a dual-band AP, so you have to configure your WLAN SSID under Configure/Radio 2.4G, and Configure/Radio 5G separately. The common settings can be left at default, and you setup encryption and SSID under Wireless 1 tab and Wireless 9 tab, respectively per radio.
The default SSID settings are None for Local Subnet, and 'Bridge to WAN' for packet forwarding.
If you do not specify a Local Subnet under the SSID, your clients DHCP request will hit the wired network, and need to obtain an address/netmask/gateway from your cable modem/DSL router, or the ISP, depending on your agreement with them. If they only give you one IP, this will not work.
The alternative that doesn't require DHCP from your local router or ISP, is to define a Local Subnet on the AP (Configure::Local Subnets), specify this local subnet under the SSID, with the 'Local Subnet NAT and Route to WAN' packet forwarding option. This is the design John was suggesting, and I wished to provide another description for.