I finally went to the office this morning and swapped my 5120 for a 6120. Had to call in for them to authorize the modem even though the office said they did that.
What's really weird is the config file that I have is d11_m_sb6120_walledgarden_c01.c which makes no sense! It's working though. I went ahead and sprung for the Extreme speed, although I don't get the full 10Mbit so far upstream so I may back down to the Ultra tier.