Are you daisy-chaining port forwarding rules, one from the modem to the router and then another one from the router to the Core? You may / probably have to. I suggest you try this:
- The modem is the external interface that gets the ARC connection from the outside and it must be able to forward from its external IP & port to the internal network, this being represented by the router. This port forwarding rule on the modem should forward the port to the router.
- Set up a similar rule on the router, so that it forwards the traffic that is coming from the modem to its incoming port, onward to the Core