Hello Gianluca,

This is not the right flow. If you would like to have any other network in your cluster as the management network, then you need to create it as the management network of the cluster and then add hosts to the cluster, then the new network which was defined as the management network of the cluster will be the management network of the host/s in the cluster.

The correct flow will be :

1) Create new network for example - 'net1-mgmt'

2) Give 'net1-mgmt'  a management role via manage networks, it can be done via networks>clusters flow or via Cluster>logical networks flow(note that management network must be required and attached)

3) Create cluster and choose 'net1-mgmt' as the management network of the cluster

3) Add/install the host in the cluster

See also - https://www.ovirt.org/develop/release-management/features/network/management-network-as-a-role/

It's a network feature since 3.6

Cheers)