Ftp and ip address

I try to use the ftp add on on an external device.
My system is installed on an ubuntu plateform via docker.
About my router, I have to forward port 20,21 to the ftp server.
I don’t know what is the local ip adress of the ftps server, is it the same as the machine ?
Best regards

A port 21-20 setup is an active FTP connection and the issue with those is that port 21 is incoming to the server while port 20 is incomming to the client.

In other words your portforwarding is faulty.

Normally you would use a passive ftp connection instead, but you need to configure the server for this.
On a passive ftp connection both connections will be incomming to the server and your portforwarding will be easier.
In a passive ftp connection one of the connections will typically be a range instead of a single port.