I too have a cable modem and successfully got online using my cable modem. I'm sure your RB750xxx comes with some sort of default configuration pre-loaded. My RB2011 did and I had to connect my my cable modem to ethernet port 1. The wireless was also enabled by default but was unsecured so I had to make a few changes to the default security profile in order to get it ip and running in a secure way.
So my guess is that you just connect your cable modem to port 1 and wait 3-4 hours to allow your ISP to auto-register the MAC address of your new RB750xxx port 1. If nothing happens after 3-4 hours and there's still no internet connection then your ISP doesn't auto-register new MAC addresses and you have to contact them to release the old and register the new. Else your default configuration doesn't use port 1 and you should refer to the manual of your 750xxx to find the information about it's default configuration.