With port 21 open, you do not have a problem, it is all down to the way you are testing it.
You will not be able to test, or use port forwarding from a device on your local network making a connection to your WAN IP address, or DDNS URL. If you did, the connection attempt would be routed out to the internet and back in again to your router. Once it comes back into your router again, it will not be forwarded to the device your port mapping rules state, but will be blocked by the router. This is known as “Loop back” traffic, which most routers will discard those packets as an additional security measure.