I'm trying to migrate a website from one server to another and I am having an FTP issue on the new server. The site has content pushed up to it by FTP by an in house piece of software, I know using FTP is wildly advised against but for the time being I have no choice in the matter. I have set up FTP apparently fine enough and can successfully transfer files using FileZilla, CyberDuck and ftp terminal clients but using our software it will login in, create the file of 0 bytes and time out (ETIMEDOUT) every time except when using the network at my office. I have ran this off 4G dongles, through VPNs, at home etc and it will fail.
The ftp client used in our software is jsftp, which only works in passive mode, so switching to active is not a possibility. The software runs on macOS.
The previous server works fine enough from any location, it is a MediaTemple shared Debian machine that used pure-ftpd. The new server I am migrating to is running Ubuntu 16.04 and hosted by Google Cloud Platform. I have used both vsftpd and pure-ftpd. I have tried using another Google Cloud server set up on my personal account and the same problems occur.
I can't figure out where the problem in this lies, the software works totally fine on the old server and when used in the office also works totally fine on the Google servers. The Google servers can also be accessed by numerous other clients and work regardless of being in the office or not. I can not find anything online about this specific situation so any ideas are welcome.