Unplug the modem from the power. Wait at least 30 seconds. Plug it back in. If the correct version of the firmware is not present, it will automatically be pushed out at that time.
When you say "The old firmware, which allowed control of the SB5100", I'm wondering what control you had. The only control that should be available to you is to reset the modem. If you had more control than that, you probably had the wrong firmware, and the correct version was pushed-out the first time you power-cycled the modem.