net.ipv6.conf.all.use_tempaddr = 0
net.ipv6.conf.default.use_tempaddr = 0
net.ipv6.conf.ens192.use_tempaddr = 0

net.ipv6.conf.default.autoconf = 0
net.ipv6.conf.all.accept_ra = 0

net.ipv6.conf.all.proxy_ndp = 1

net.ipv4.ip_forward=1
...
net.ipv6.conf.all.forwarding=1

network:
  version: 2
  renderer: networkd
  ethernets:
    ens18:
      dhcp4: no
      addresses:
        - a.b.c.d/24
      gateway4: a.b.c.e

      dhcp6: no
      accept-ra: no
      addresses:
        - "2001:0db8:0004:4a1a::dead:beef/64"
      gateway6: "2001:0db8:0004::0001"

    ens19:
      dhcp6: no
      accept-ra: no
      addresses:
        - "2001:0db8:0004:4a1a::dead:dead/64"
      gateway6: "2001:0db8:0004::0001"

$ ip -6 addr show

2: ens18: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP qlen 1000
    inet6 2001:0db8:0004:4a1a::dead:beef/64 scope global
       valid_lft forever preferred_lft forever
    inet6 fe80::abcd:efff:fea3:2a03/64 scope link
       valid_lft forever preferred_lft forever
3: ens19: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP qlen 1000
    inet6 2001:0db8:0004:4a1a::dead:dead/64 scope global
       valid_lft forever preferred_lft forever
    inet6 fe80::defc:deff:fe98:7c74/64 scope link
       valid_lft forever preferred_lft forever

2001:0db8:0004:4a1a::/64 dev ens19 proto kernel metric 256 pref medium
2001:0db8:0004:4a1a::/64 dev ens18 proto kernel metric 256 pref medium

fe80::/64 dev ens19 proto kernel metric 256 pref medium
fe80::/64 dev ens18 proto kernel metric 256 pref medium

$ ping -6 -c3 ipv6.google.com
connect: Network is unreachable

$ sudo ip -6 route add 2001:0db8:0004::0001 dev ens18 metric 1
$ sudo ip -6 route add default via 2001:0db8:0004::0001 dev ens18 metric 1

2001:0db8:4::1 dev ens18 metric 1 pref medium

2a0b:ae40:4:4a1a::/64 dev ens19 proto kernel metric 256 pref medium
2a0b:ae40:4:4a1a::/64 dev ens18 proto kernel metric 256 pref medium

fe80::/64 dev ens19 proto kernel metric 256 pref medium
fe80::/64 dev ens18 proto kernel metric 256 pref medium

default via 2001:0db8:4::1 dev ens18 metric 1 pref medium

$ ping -6 -c3 ipv6.google.com
PING ipv6.google.com(ord38s18-in-x0e.1e100.net (2607:f8b0:4009:804::200e)) 56 data bytes
64 bytes from ord38s18-in-x0e.1e100.net (2607:f8b0:4009:804::200e): icmp_seq=1 ttl=56 time=23.2 ms
64 bytes from ord38s18-in-x0e.1e100.net (2607:f8b0:4009:804::200e): icmp_seq=2 ttl=56 time=23.2 ms
64 bytes from ord38s18-in-x0e.1e100.net (2607:f8b0:4009:804::200e): icmp_seq=3 ttl=56 time=23.2 ms

--- ipv6.google.com ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2002ms
rtt min/avg/max/mdev = 23.274/23.283/23.293/0.176 ms