Ok, now I have deployed ISPConfig 3.1.13p1 on an Ubuntu 18.04. I also have enabled FTP TLS on pure-ftpd. I set its port range from, for example, 2000 to 2100. Do I still have to open TCP port 21? I heard that we also have to open TCP port 20 in this case. Is it true?
Yes. The other ports 2000 - 2100 are the so called passive port range and are used in FTP passive mode.