It's by design, so that you don't go flooding the internet with spam and are forced to use the ISP's SMTP server instead (where they have control of whether you spam people or not).
Really it's daft to block you from using port 25 to send email via your own server
which requires authentication but that's what some WiFi ISPs seem to do.
The solution is to play hide-and-seek with different port numbers.
Find a port other than 25 on which your SMTP server will play and use that, and hope that the WiFi ISPs don't spot people doing that and block that one too.