Can't connect to FTP server when I'm connected via hotspot

Being older than IP networking itself (yes, really), and its pattern of connections is fundamentally incompatible with modern network firewalls, and being wildly insecure, FTP quite often causes issues, and increasingly often simply gets blocked.

To get FTP to work with a firewall, one or both firewalls (on the local network, and in the remote network) must scan the FTP traffic and preemptively open up the second port. The direction of this second connection depends on whether the FTP connection is operating in active or passive mode. But it’ll always run into a firewall, if the firewall isn’t configured to scan FTP traffic.

(This second data connection used by FTP usually uses an ephemeral port selected at run-time, so there’s no way a firewall can predict that without communications with the FTP server or FTP client, which means either opening up the entire ephemeral range, or configuring the FTP server to use a particular sub-range and opening that, or (as is common on middling and higher-spec firewalls) scanning the passing FTP traffic.)

As FTP openly leaks the server credentials in cleartext, the recipient firewall and recipient FTP server need to process and relocate the arriving files too, as the server can otherwise become filled with unwanted or illegal files.

If you need file uploads with your web server, use HTTPS file uploads, or configure and use SFTP.

If you really need FTP, set up a VPN to wrap the connection; that VPN might be L2TP/IPSec, or maybe FTPS.

PS: also learn about Gifar and other types of polyglot files, particularly if you’re exposing potentially untrusted uploaded files.