I've an /29 IP subnet provided by our ISP. I've tried to configure port forwarding for specific port xx using a specific WAN IP alias, it works, I'm able to connect from external to the specified port just on the specified WAN IP alias. But If I want to specify the same specific port xx using a different WAN IP alias, I'm not able to save the config, as the system mentioned that the port is already in use by the interface. Is the router (ER605) or the omanda controller not flexible enough to handle such situation ?

I'don't want to use 1 to 1 NAT in such case, as I would not be able to forward different ports of specific WAN IP alias to different servers in the private range