FTP client connection to readyNAS FTP Server keeps disconnecting

Hello Support,

I need your help to fix ftp disconnection issues.

My company have office based at East Europe and one at China. Europe users are using ftp client to transfer files to China's readyNAS FTP server. They are ok when transferring small size of files (<100MB) but keep experiencing ftp disconnection issues when transferring big size of files (up to or bigger than 1GB). We tried different ftp clients and none of them worked properly.

I still cannot figure it out if that is caused either from China's Great Firewall interuppting the connection or China's readyNAS FTP server side (note: I'm talking about transferring large files) as most of the errors we got were the connection failed  due to remote server closed or rejected the connection. We had a failed transfers on files that reached 99% transfer and then failed.

Both offices are VPN IPsec connected.

Please let me know for any alternative solutions to make ftp work.

Thank you in advance.