Republished with permission from the author Kevin Scott, and with thanks!
I have successfully tested a Huawei USB 3G modem (K3765 Unlocked) with the RB951 FW 6.31 as a second internet link in fail over mode.
I have 2 x internet connections, 1 x ADSL2+ via IINET and the other being 3G with Telstra Pre Paid. The IINET ADSL is the prime service and the Telstra 2G is secondary.
If the ADSL goes down, within a few seconds the 3G is providing the internet connectivity. The device is very easy to get working and doesn't require too many settings other than the standard - an apn of telstra.internet, ATDT for the dial command, *99# as the phone number, a masquerade in the firewall tables, a route in the routes tables and obviously the sim to be prepaid and activated. PPP Info
Firewall - Nat
Route with fail over to secondary internet
The above works flawlessly with an unlocked 3G Huawei K3765 Dongle with a 30 second or so automatic fail over for change of routing occurrence.