Followed the instructions from https://access.redhat.com/solutions/648863 (https://access.redhat.com/solutions/6...)

Now the instances are accessible outside the overcloud aswell.

LE: requested config details:

[heat-admin@overcloud-novacompute-0 ~]$ sudo cat /etc/neutron/l3_agent.ini | grep -v ^# | grep -v ^$
[DEFAULT]
[AGENT]
[heat-admin@overcloud-novacompute-0 ~]$ sudo cat /etc/neutron/plugins/ml2/openvswitch_agent.ini  | grep -v ^# | grep -v ^$
[ovs]
integration_bridge = br-int
tunnel_bridge = br-tun
local_ip =192.0.2.8
bridge_mappings =datacentre:br-ex,physnet1:br-ext
enable_tunneling=True
[agent]
polling_interval = 2
tunnel_types =vxlan
vxlan_udp_port =4789
l2_population = False
arp_responder = False
prevent_arp_spoofing = True
enable_distributed_routing = False
extensions =qos
drop_flows_on_start=False
[securitygroup]
firewall_driver = neutron.agent.linux.iptables_firewall.OVSHybridIptablesFirewallDriver

[root@overcloud-controller-0 ~]# cat /etc/neutron/l3_agent.ini | grep -v ^# | grep -v ^$
[DEFAULT]
debug = False
interface_driver =neutron.agent.linux.interface.OVSInterfaceDriver
ovs_use_veth = False
handle_internal_only_routers = True
external_network_bridge =
metadata_port = 9697
send_arp_for_ha = 3
periodic_interval = 40
periodic_fuzzy_delay = 5
enable_metadata_proxy = True
router_delete_namespaces = True
agent_mode = legacy
[AGENT]
[root@overcloud-controller-0 ~]# cat /etc/neutron/plugins/ml2/openvswitch_agent.ini  | grep -v ^# | grep -v ^$
[ovs]
integration_bridge = br-int
tunnel_bridge = br-tun
local_ip =192.0.2.9
bridge_mappings =datacentre:br-ex,physnet1:br-ext
enable_tunneling=True
[agent]
polling_interval = 2
tunnel_types =vxlan
vxlan_udp_port =4789
l2_population = False
arp_responder = False
prevent_arp_spoofing = True
enable_distributed_routing = False
extensions =qos
drop_flows_on_start=False
[securitygroup]
firewall_driver = neutron.agent.linux.iptables_firewall.OVSHybridIptablesFirewallDriver
[root@overcloud-controller-0 ~]# ovs-vsctl show
83bba490-eb01-48f3-986c-f99281c8f5fc
    Bridge br-ext
        Port phy-br-ext
            Interface phy-br-ext
                type: patch
                options: {peer=int-br-ext}
        Port br-ext
            Interface br-ext
                type: internal
        Port "ens7"
            Interface "ens7"
    Bridge br-int
        fail_mode: secure
        Port br-int
            Interface br-int
                type: internal
        Port "qg-7ab64067-19"
            tag: 3
            Interface "qg-7ab64067-19"
                type: internal
        Port int-br-ext
            Interface int-br-ext
                type: patch
                options: {peer=phy-br-ext}
        Port "qr-a463dbc5-dd"
            tag: 1
            Interface "qr-a463dbc5-dd"
                type: internal
        Port "tap611e1a4a-a9"
            tag: 1
            Interface "tap611e1a4a-a9"
                type: internal
        Port patch-tun
            Interface patch-tun
                type: patch
                options: {peer=patch-int}
        Port int-br-ex
            Interface int-br-ex
                type: patch
                options: {peer=phy-br-ex}
    Bridge br-ex
        Port "eth0"
            Interface "eth0"
        Port phy-br-ex
            Interface phy-br-ex
                type: patch
                options: {peer=int-br-ex}
        Port br-ex
            Interface br-ex
                type: internal
    Bridge br-tun
        fail_mode: secure
        Port "vxlan-c0000208"
            Interface "vxlan-c0000208"
                type: vxlan
                options: {df_default="true", in_key=flow, local_ip="192.0.2.9", out_key=flow, remote_ip="192.0.2.8"}
        Port patch-int
            Interface patch-int
                type: patch
                options: {peer=patch-tun}
        Port br-tun
            Interface br-tun
                type: internal
    ovs_version: "2.5.0"
[root@overcloud-controller-0 ~]# ifconfig
br-ex: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.0.2.9  netmask 255.255.255.0  broadcast 192.0.2.255
        inet6 fe80::293:7cff:fedb:87ea  prefixlen 64  scopeid 0x20<link>
        ether 00:93:7c:db:87:ea  txqueuelen 0  (Ethernet)
        RX packets 502303  bytes 590484296 (563.1 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 857201  bytes 102417125 (97.6 MiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

br-ext: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 10.8.75.242  netmask 255.255.255.0  broadcast 10.8.75.255
        inet6 fe80::2c29:73ff:fe21:2941  prefixlen 64  scopeid 0x20<link>
        ether 2e:29:73:21:29:41  txqueuelen 0  (Ethernet)
        RX packets 70369  bytes 7646523 (7.2 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 102  bytes ...