As of the time of writing, the conjure-up installation does not create a physical network that can be reached from the outside.

To fix this, change the conjure-up neutron-gateway parameter data-port to br-ex:ens8. This will bridge the OpenStack physical network physnet1 to the second interface on the neutron-gateway service host, putting your allocated floating IPs on whatever subnet is available at that port.

You can also adjust this after a conjure-up installation by running from the MAAS host:

juju run --unit neutron-gateway/0 sudo ovs-vsctl add-port br-ex ens8

However, this won't be recognized in the neutron-gateway charm settings and probably won't work long term. I'm not yet sure how to adjust and apply settings changes after deployment.

Your OpenStack public network will need to have the following parameters:

Admin State: Up
Shared: No
External Network: Yes
Provider: Network Type: flat  # Unless the second port is connected to a VLAN switch
Provider: Physical Network: physnet1

The subnet should be the same as that of the second port of the neutron-gateway service host. For a conjure-up install as described in the Canonical documentation (https://ubuntu.com/openstack/install#...), it will be the same network as the MAAS server. Disable DHCP on the OpenStack subnet, otherwise OpenStack will be fighting with MAAS.