this is for my homeserver.
I already figured it out, I did not install the cockpit-ws package…
Correct way of doing it:
rpm-ostree install cockpit-system cockpit-ws cockpit-files cockpit-networkmanager cockpit-ostree cockpit-podman cockpit-selinux cockpit-storaged
sudo systemctl enable --now cockpit.socket
sudo firewall-cmd --add-service=cockpit
sudo firewall-cmd --add-service=cockpit --permanent
Now it works at localhost:9090 
I should not have followed instructions for CoreOS as that is container-based OS… my mistake!