Using BigIP as a load balancer as well as a router

The routing table provides the path, but a virtual server (or a default snat which behaves similar) must exist for the traffic to flow. You can configure the default virtual server to only apply to your subscribes vlan, and you can limit to a port, or configure a few virtuals, applied only to subscribers & the ports they need to reach. If that is excessive, you can stick with port 0.