You'll need to launch your instances on an internal network so that they can access dhcp and metadata. External networks are there to use for Floating IP addresses.

Floating IP > NAT > Internal IP

Public networks should be created in the Admin tenant.

You'll want to work out of another tenant, say create one called "demo" like the guides, or create a tenant based on the project you are working on. From that point you can create an internal network for the tenant. You'll launch the instance on the internal network in the new tenant.

The issue with using the Admin tenant is that you're able to see all of the resources available. You aren't meant to launch instances on an External network (at least not out of the box.) DHCP is supposed to be disabled on the Public Network as well (external network.)