You can configure the router to provide NetInstall service across subnets.
A network boot starts when the client computer broadcasts for computers that will respond to Boot Service Discovery Protocol (BSDP). By default, routers are usually configured to block broadcast traffic to reduce the amount of unnecessary data flowing to other parts of the network.
To provide NetInstall service across subnets, you must configure the router to pass on BSDP traffic to the NetInstall server. To see if your router is capable of passing BSDP traffic, check with your router manufacturer. Sometimes this is also referred to as using a DHCP helper or a DHCP relay agent.