If you didn't configure your router correctly you will get no port unreachable -> it will claim port open.
UDP scans: This method is used to determine which UDP (User Datagram Protocol,
RFC 768) ports are open on a host. The technique is to send 0 byte udp packets to
each port on the target machine. If we receive an ICMP port unreachable message,
then the port is closed. Otherwise we assume it is open.
If everything is configured correctly, you will not get the icmp port unreachable message -> it will claim port open.