In OpenStack you are the admin of your cloud, so you can create a public network in OpenStack. Follow the below given steps.
Log in to the OpenStack dashboard as an administrator.
Verify the existence of external networks. For this, navigate to Project > Network > Network Topology.
Check the network type in network details. For this, navigate to Admin > Networks and see the Network name section. Alternatively, run the openstack network list --external command using CLI.
Navigate to Project > Network > Networks>create network
Hope these steps help you.