I am now up on the new (registered) modem and all is well...up/down speeds are what I expect.
Because there was some discussion here (thank you for the comments) about what should/should not have worked, I will confirm that the new modem (SB6121) did work prior to MAC registration...it just works better now.
Tonight I reconnected the new modem, and prior to registration and modem restart, while speeds were slow, they were good enough that I used my VOIP phone service (Ooma) to make the call to WOW support. Call quality was fine.
Now I'm hoping changing to the SB6121 modem will fix the voice phone call issue I was having. I just switched to WOW from DSL and VOIP quality fell off with the Arris.