He got an actual router from Comcast? If so, he'll need to set up port forwarding from the Comcast rtr to the 300N.
I find it weird that a standard business package doesn't come with at least one fixed IP. You'll have to run DDNS on either the Comcast rtr or a device inside the network. Also, if you don't have a fixed IP, Comcast may not allow post 80 or 443 traffic TO the Comcast rtr. You could look it up, or contact Comcast support to make sure they aren't blocking. If they aren't, you would just forward 443 to the WAN IP of the 300N (which is internal to the Comcast rtr). You'll want to statically assign that address to the 300N. On the 300N, enable web management using https. This is under Administration->Admin Access->Remote Access (select HTTPS, change port to 443).
IF Comcast doesn't allow 443 (HTTPS) traffic, you'll have to use a non-standard port (I like 444, because it's easy. Some may recommend 8080, but I use that for non-standard http access). On the Comcast rtr, you'll have to forward the nonstandard port instead of 443 to the internal address of the 300N.
Why is he running a router inside of the network? What is he trying to accomplish?