I spent a bit of time struggling with this last week when my Cellspot arrived, and figured I would pass this short tutorial on to the community in case they need it.
If you have Comcast Business Internet services (rather than the cheaper but less flexible XFINITY service) you know that rather than providing a simple cable modem they give you a 4-port gigabit router. This router, while serviceable for basic use, lacks most of the features found in the Cellspot. Additionally, if you put your Cellspot behind the Comcast router, you will run into issues with NAT on systems that go through both.
To remedy this, you can either buy your own cable modem from a very small list that are supported by Comcast for use on their business service, or you can do the following:
1. Log into your Comcast router by accessing it on your LAN through a wired connection. The default IP address is 10.1.10.1, and the default username and passwords are cusadmin and highspeed.
2. Go to the LAN menu (on the left side menu), and uncheck the box for Enable LAN DHCP. Hit the Apply button.
3. Go to the Firewall menu (again, on the left side menu). Under the DMZ tab on the Firewall menu check the Enable DMZ Host, and then put an appropriate address in the box. 10.1.10.2 is a good choice. Hit the Apply button, and log out of your Comcast router.
4. Connect your Personal Cellspot to one of the ports on the back of the Comcast router, and log into your Personal Cellspot router via the instructions given with it. It's best to use a wired connection here as well if possible.
5. On the Cellspot router management interface, go the the WAN menu (from the left side menu).
6. On the Internet Connection tab, set the WAN Connection Type to Static IP, and then under the WAN IP Setting section put in the IP Address you used in step 3. For Subnet Mask use 255.255.255.0, and for the Default Gateway use 10.1.10.1. Hit the Apply button and wait for the setting to apply.
7. Move any devices you have connected to the Comcast router's ports to the ports on the back of the Personal Cellspot (except the Cellspot itself, of course). You may need to reboot all of the devices connected to your LAN or WLAN to get them to work right.
With this completed, your Comcast router essentially becomes a modem and you may now use the substantially more powerful LAN features of your Cellspot router instead.
Any questions, feel free to ask.