IPV6 Setup 3939B and Unifi USG

Attempting to setup IPV6 on a Unifi USG 4Pro version 5.11.50. I have a 3939B from Comcast, and I have been told it's the most up to date modem adn should route IPv6 to my firewall. I have a static IPV4 block and static IPV6. I'm trying to set the USG as a static IP.

For my WAN setup I'm asked for the following:

IPV6 Address - I'm using the IPV6 address located on my account page.

Prefix Length - Based on Comcast help documentation, I believe it's 64 even though my account page shows 56.

Router - I'm currently populating the field with the WAN IP Address from the modem WAN Status webpagebut there are other fields that might be right: WAN Default Gateway address fe80::b290:xxx and a delagated prefix which is the same address that shows on my account page. I'm currently populating the field with the WAN IP Address. There's also a WAN link local address.

For my LAN setup I'm asked for the following

IPv6 Gateway/Subnet - what data should go here? Is this my delegated prefix? Based on what I enter, the webpage for the USG/Unifi prepopulates a DHCP6 range. And I'm told hte lease range has been limited to 2000 leases.

I would really appreicate any help.

Thanks.