DHCP6-PD seems to be ignored?

I have a Cisco DPC3939B modem/router from Comcast, and am trying to get the IPv6 support working.

I would like to have it delegate me a /64 from the /56 the website lists, but can only get it to hand out single IP addresses.  The PD request appears to be ignored.  To get IPv6 working I've had to set up a tunnel through Hurricane Electric, which is a ridiculous thing to do in 2020.

I see a bunch of people here with the same issue, and never see a response, just "contact me directly".  I'll do that if needed, but I'd rather a FAQ or searchable document be in place so I didn't have to struggle with what appears to be a normal configuration.

I am trying to do the DHCP6-PD because that sounds like the "right" answer, but I'd be happy to configure static IPs as well, but when I do, the device won't route them and they never leave my local LAN.

I have static IPv4 addresses, so apparently have to leave the device in router mode and can't just use bridge mode and get it to stop "helping".

Why is this so difficult, and what do I need to do to get it resolved?