Check your version of the RUT240 firmware - I think support for Verizon was added in 1.12.2.
I have also had some issues independent of the firmware - sometimes, when I insert a new SIM card, I have to click on the Reconnect button in the Mobile Networks tab in the browser, and that seems to work.
Finally, make sure that the Mobile Network, and not the WAN port, is listed as the primary "failover" - this one got me into trouble as well. Oh - and don't pick the load-balancing option unless that is what you intend to have.