For more information including pictures of the CTR-350 that I'm using, see the following web page:
http://www.cradlepoint.com/ctr350/ctr350.php
The internet service comes over the Sprint cellular network and I'm using a Sierra Wireless Compass 597 USB modem:
http://www.sierrawireless.com/product/compass_597.aspx
This USB modem plugs into the USB port on the CTR-350 which converts the cellular packets into Ethernet packets. The CTR-350 is a router and has built-in WIFI, but I have disabled the WIFI and I have set up the DMZ with the WAN IP address of the Cisco ASA firewall. This means that all traffic should be bypassing any of the "router" features of the CTR-350.
The Ethernet port on the CTR-350 can operate in a WAN or LAN mode -- I'm using it in LAN mode so that it can connect to the WAN port of the Cisco ASA.
I actually ended up configuring the Cisco ASA (using the serial cable and the provided instructions) to use a static IP address for its WAN port -- the reason being that I couldn't seem to get the Cisco ASA to get an IP address using DHCP to the CTR-350. Any other computer I plugged into CTR-350 easily got an IP address using DHCP, so I don't know why I had troubles with the Cisco ASA. I left the DHCP server running on the CRT-350 in case I ever want/need to trouble shoot by plugging in a computer instead of the Cisco ASA.
Orginally, we were being steered towards the Cradlepoint MBR-1000, but it was $100 more and it had more features (i.e. 3 external wifi antennas, 4-port switch, etc.) that we weren't going to be able to use anyway (because it was in front of the firewall)...so that's why I went with the CTR-350 instead. It was there least expensive unit with the minimum features of having at least a USB port and one Ethernet port.
Let me know if you have any more questions.
Rob