What is the configuration you use in the iPhone app?
The file manager itself uses the same FTP server as your mobile app does. So if the file manager works, likely FTP is working for your account as well. Which makes me suspect that the configuration of your app may be the issue.
interesting thing is I didn’t change the settings from the beginning. It is the same. It just worked for 1 day only and next day stopped.
server : ftp.epizy.com
login : my username
pass: my password
director : I have chosen the correct one under the root, I know root cannot be overwritten.
port : 21
ftp mode: automatic