Every modem provided by Comcast supports dual stack broadband and IPv6 only for management by default. The latter is transparent to customer and is for internal use only. IPv6 only for management has no impact on dual stack broadband. If your modem is in bridge mode (Wi-Fi router functionality disabled) then you need to ensure that your broadband router supports IPv6 specifically DHCPv6 for the acquisition of IA-NA and IA-PD.
HTH,
John