For the STB connection you need a PID number and Password
For the Cable Modem you need a Pin number and your Surname
The STB connection is billed on a system that can be paid either DD, Cheque, Cash etc (is billed together with TV and Telephone if you have those as well)
The Cable Modem billing system has to have either a DD or Continous Credit Payment active on the account before it will let you register.
I would recommend that you have a Cable Modem for your connection, CS
should be able to give you the pin number you need for that, check you have an active payment method and get you sorted, registered and up and running, this is easy for them to sort out

They should only ensure that once you have registered that the charge for the STB BB is removed so you are only paying for one BB service.