The debugger is actually an HTTP server that works with websockets, if I’m not mistaken:
A Dart VM Service on iPhone 16e is available at: http://127.0.0.1:50927/OsKCWOsj3gs=/
The Flutter DevTools debugger and profiler on iPhone 16e is available at: http://127.0.0.1:9101?uri=http://127.0.0.1:50927/OsKCWOsj3gs=/
Your device must be able to connect using those endpoints. Maybe something is preventing it.
Another thing you should be aware: as usual, Apple f*** up us, developers: PSA: Do not update your development devices to iOS 18.4 - #2 by johnpryan
Check if you are using iOS >= 18.4
EDIT: From your phone, you should be able to call that http://address:50927/ and it should display a “missing or invalid authentication code”. If the iPhone can’t reach that URL, the debugger won’t be able to connect.