As you have already discovered, the RBR750 is intended to be connected to a coax cable (and is compatible with Xfinity). There is no option to connect it to the Xfinity I do not understand why use of the Xfinity modem is a requirement.
Just to double check, navigate to the Orbi Advanced Tab, Advanced Settings, and see if there is an option to place the RBR750 in Access Point (AP) mode.