iOS & Android Mobile App Development

We are developing a web-based TASK PLANNER application. We want to develop an application that runs on iOS and Android Platforms that will communicate with REST via this application.The app will include items like list and instant messaging. Detailed documents will be given to interested people. I do not want to web frame application. Some developers integrate my website responsive design into the app. I can do that I want mobile app.

We want one of these (flutter / swift / java / objective-c)

Example:
https://www.behance.net/gallery/106510451/Taskito-Task-Management-App?tracking_source=search_projects_recommended%7Ctask%20management%20ui%20mobile