OK, to augment the netplan configuration you need to add a or edit an existing .yaml file in
/etc/netplan/. The files are considered in order and the ones with higher numbers override those with lower.
My cloud provider created a default .yaml
/etc/netplan/50-cloud-init.yaml which I left as is and added a
/etc/netplan/99-static-network.yaml which augments the provider's config:
Then I created the config files from my yamls with
netplan generate and applied the changes with