I have the U600 modem working fine with Windows XP and the Sprint host software.
When I plug it into the 3420 router the router finds the USB Modem and identifies it and then it says that the APN or dialing number is wrong. The router thinks the dialing number is 777# but it doesn't tell me what APN it is trying to use. I punched in a couple of APNs mentioned in this forum with no luck.
If this would work it would be a very low cost way to get a wired ethernet network connected to the internet via Ting. The modem was $5.00 via ebay. The Router was about $35 via Amazon.