If you encounter error 503 when trying to log in through the API, it means that you are using an outdated API URL.: https://api.fyers.in/vagator/v1/, which is no longer supported.
You need to replace it with the latest endpoint that is mentioned in the documentation and follow the steps, Link: https://myapi.fyers.in/docs#tag/Authentication-and-Login-Flow-User-Apps/paths/~1Authentication%20&%20Login%20Flow%20-%20User%20Apps/post
For more details, please refer to the documentation, Link: API - FYERS