Comment 31 for bug 1609898

Also note that dnsmasq can act as both a dhcpv6 server as well as serving router advertisements.

The ISC dhcp server only does dhcpv6, it does not have support (that I know of) for serving router advertisements.

If you do want your clients to use slaac addresses in addition to the dhcpv6 address, change:
  AdvAutonomous on; # clients will use SLAAC (RFC 4862) addrs

If you do want your server to act as the client's default router (gateway), remove:
  # AdvDefaultLifetime 0; # this server is a default router (gateway) when commented
or change the value to a non-zero number of seconds