Why instances are not able to reach other networks outside of external network?

Issue

• I have created an external network with below details.

+------------------+-----------------------------------------------------+
| Field            | Value                                               |
+------------------+-----------------------------------------------------+
| allocation_pools | {"start": "10.5.11.1", "end": "10.5.11.20"}         |
| cidr             | 10.5.0.0/16                                         |
| dns_nameservers  |                                                     |
| enable_dhcp      | False                                               |
| gateway_ip       |                                                     |
| host_routes      | {"destination": "0.0.0.0/0", "nexthop": "10.5.0.1"} |
| id               | 8aed580a-af5d-46c8-9c7f-32124b35db65                |
| ip_version       | 4                                                   |
| name             | roc_net                                             |
| network_id       | adf0c0c8-b6ef-437e-905a-1f557f631d77                |
| tenant_id        | 40f8e06a04f84a67ad74658d4ef6bfda                    |
+------------------+-----------------------------------------------------+

Then created instances to a private network which is connected to the external network using a neutron virtual router. I then assigned a floating ip to the instance. Instance can access all the nodes from the external network, but cannot access systems from any other network beyond the external network which are accessible using the cisco gateway/router available in the external network.

How can I resolve this?