You'd need to forward the ports on both routers to pass the traffic through. e.g., port 5678 on the outside of the D-Link would point inside to the outside IP of the Asus which would have port 5678 forwarded inside to port 5678 on the PC. Or you may be able to bridge the first router. Or you could flatten or reorg your network to make it easier, do it via VPN, etc.