There is an official guide regarding ipv6 configuration using Ubuntu’s netplan. It seems it skips one crucial thing tho - and that is default route. The route step is mentioned in non-persistent and file interfaces sections (the ip -6 route add part). In order to fully get v6 working on their VPS I had to add the route via routes declaration in netplan yaml configuration (Ubuntu LTS 18.04).

# /etc/netplan/51-cloud-init-ipv6.yaml
    version: 2
      ens3: # you main eth device
        dhcp6: false
          name: ens3 # same ^
          # ipv6 address can be obtained from their web ui
          # netmask offered by ovh on their VPSes weirdly is not mentioned explicitly anywhere
          - "YOUR_IPV6/128"
        gateway6: "IPv6_GATEWAY" # v6 gateway can be obtained from their web ui panel
          - to: "IPv6_GATEWAY" # same as above ^
            scope: link # this should work just fine on modern (v0.98+) netplan

Run netplan apply after introducing changes and try ping v6 host or simply execute some http request:

curl -v -6