If networks are attached to a router using the 'router-interface-add' command, static routes are not necessary. Traffic between instances in those different networks via their fixed IPs is allowed by default provided the appropriate security group rules are in place. It may be necessary to perform packet captures on the computes/network nodes to see if/where packets are being dropped and if there are any other issues.

look like you need to write static routes inside the router ' that you can write through horizon'