Hi,
no idea if that is required for your ISP, but I have two options in my
working pppd config related to IPv6:
+ipv6
ipv6 ,
The second line is ipv6[space][comma]. See "man pppd". It means that
local and remote interface identifiers are randomized.
You don't mention any DHCP clients. At least for me, I need a DHCPv6
client to get the IPv6 working. I get an IPv4 address from the ISP
directly via ppp, but to get a globally-routable IPv6 requires a DHCPv6
client.
I use the client in the "wide-dhcpv6-client" package.
Hope that helps,
Tomaž