Fail to get more than on /64 from /56 static business account - need some help

Hi - am a bit new at ipv6 so I'm quite likely missing something.

Account is static business acount- ipv4 all working well - no issues.

The router provided labels itself: comcast hardware:  Model:CGA4131COM  Vendor:Technicolor

I've been delegated a /56 (xxxx.yyyy.zz::/56) - my firewall gets a an ip from a /64 address (xxxx.yyyy.zz00:aaa.bbb.cccc.dddd) as well as a link local fe80:. The firewall can connect and use the internet on both ipv4 and ipv6 just fine.

Next step is where I get stuck. I started a DHCP-PD (dhcpcd) to request 1 (or 2) additional /64 prefixes to use on the internal network - best I can tell no other /64 is delegated and so I have no prefix to work with on the internal side. 

I tried choosing a second /64 out of the /56 (xxxx.yyyy.zz01::/64)  - assigning IP's to internal client and the internl side of the firewall - doing that the clients can ping the firewall inside and its external interface which connects to the router but the packets are not routed by the CM / router to the internet - I assume the CM/router wont route any additional /64 until it knows about them.

I logged in to the CM/router but I was unable to find any way to set up static routes for IPV6 for the additional /64.

Question - how do i get the CM/router to route additional /64 subnets?

thanks for your help.

gene