Configuring additional ipv6 address in Ubuntu 18.04

I've created a new Linode with Ubuntu 18.04 and I'm trying to follow this guide to add an additional ipv6 address: https://www.linode.com/docs/networking/linux-static-ip-configuration/

I've turned off auto-configured networking.

My Linode's public ipv6 address is: a:b::/64

Linode says my public IP pool is: c:d::/64 routed to 1:2:3::

Per the guide I've edited /etc/systemd/network/05-eth0.network and added the following lines:

# Gateway as defined by Linode
Gateway=fe80::1
# ipv6 assigned by Linode
Address=a:b::
# Additional ipv6 from pool
Address=c:d::1

Then I've rebooted the Linode.

When the Linode finishes booting, the output of ip -6 address is:

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 state UNKNOWN qlen 1000
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 state UP qlen 1000
    inet6 c:d::1/64 scope global 
       valid_lft forever preferred_lft forever
    inet6 a:b::/64 scope global dynamic mngtmpaddr noprefixroute 
       valid_lft 2591997sec preferred_lft 604797sec
    inet6 e:f::/64 scope link 
       valid_lft forever preferred_lft forever

So the new address has been assigned. But,

• From outside the Linode, I can ping6 a:b::, the original ipv6 assigned by Linode.
• From outside the Linode, I cannot ping6 c:d::1, the ipv6 I tried to add.
• From inside the Linode, I cannot ping6 ipv6.google.com.

So I broke something but I'm not sure what.

If I remove the line Address=a:b:: (the original ipv6 assigned by Linode), I get the same result. (Can ping original ipv6, cannot ping additional one.)

What am I missing here? I'm unfamiliar with Ubuntu 18.04's new way of assigning addresses.

Thanks!