Yes, you can still use Roqos VPN IN even when your Roqos Core is behind another router, which is a scenario also called "Double NAT". If your upstream router has the UPNP feature enabled, you don't need to do anything as Roqos Core will automatically instruct your upstream router to forward necessary ports to itself.
If your upstream router does not have a UPNP feature or if you disabled it intentionally, you need to put a port forwarding rule on your upstream router so that it can pass the VPN traffic to Roqos Core. By default, the VPN port is UDP 5052. If you changed the protocol or port number in the Roqos VPN settings, you will need to note those down and forward them accordingly on your upstream router.