Could it be, that the port being forwarded conflicts with another rule in the firewall?
Perhaps you could try setting the external port to something in the 50xxx range, to make sure it does not conflict with any other devices?
Also, make sure you are not testing this port forward while in the LAN of RUTX11, as RFC1918 is being enforced by default. If you'd like, it can be disabled by navigating to System → Administration → Access Control and disabling the "Ignore private IPs on public interface" option.
Best regards,
DaumantasG