I suspect you are having a firewall issue. You will need to verify that the ports being used for iSCSI traffic are allowed end to end through the firewall. Further if you are using a non standard MTU size (1500) then this needs to also be enabled end to end.
As a side not it isn't a recommended approach to route iSCSI storage traffic. I know that sometimes we need to in order for the solution to work, but we should try to avoid it if we can.
Further reading;