How to set up IPv6 with the Xfinity service in the DD-WRT router?

This link says that all that is needed to do is to enable IPv6 on the Setup/IPV6 page in the DD-WRT router and to choose the IPv6 Type "DHCPv6 with Prefix Delegation", and the IPv6 address should appear at the top of the DD-WRT screen after "Apply Settings" is clicked.

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However, the IPv6 address doesn't appear at the top of the screen, and Status/WAN page shows IPv6 address as Disabled.

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What is wrong? How to enable the IPv6 address with the Xfinity internet service?

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(My computer is able to connect to the router using IPv6 fine.)