IPv6 on Ubuntu - ping6: Network is unreachable

I've just started setting up one of my two VPS's from OVH. I have changed the hostname and reverse DNS for the included IPv4 address and done the same for the IPv6 address.

I've configured netplan with the static IPv6 address and it accepted it just fine, the address appears as expected when running `ip addr`. I am able to ping to and from the VPS using IPv4 but when trying to ping IPv6 with `ping6` from the VPS I get an output of `ping6: connect: Network is unreachable`.

I have used a couple of guides, both from OVH and in general for Ubuntu and am not able to connect with this IPv6 address.

Current contents of `50-cloud-init.yaml`: (I've also tried separating IPv6 into a separate YAML file, as expected, this makes no difference.
```
network:
version: 2
ethernets:
ens3:
dhcp4: true
match:
macaddress: REDACTED
mtu: 1500
set-name: ens3
dhcp6: no
addresses:
- REDACTED/128
gateway6: REDACTED
```

Output of `ip addr`:
```
2: ens3: mtu 1500 qdisc fq_codel state UP group default qlen 1000
link/ether REDACTED brd ff:ff:ff:ff:ff:ff
inet REDACTED/32 scope global dynamic ens3
valid_lft 84862sec preferred_lft 84862sec
inet6 REDACTED/128 scope global
valid_lft forever preferred_lft forever
inet6 REDACTED/64 scope link
valid_lft forever preferred_lft forever
```

On a slightly related note, while I'm here; is it possible to add additional IPv6 addresses?

Thanks in advanced,
Fred.