Fix My Flutter Auto Messaging App

I have an app on a VPS server that sends automated messages to clients. It appears to be experiencing API connectivity problems. The app was developed with Flutter and is hosted on Ubuntu. I have control panel access to the server and the API documentation, but I do not have access to the database.

Key Requirements:
- Diagnose and fix the API connectivity issues
- Ensure the app can send auto messages as intended
- Optimize the app for seamless performance

Ideal Skills and Experience:
- Extensive experience with Flutter and Ubuntu
- Proficient in troubleshooting API connectivity problems
- Previous work with similar auto messaging applications
- Ability to work with limited access and documentation