React Native Developer for Mobile App Development (Payment Flow Fixes)

We're a small startup that built a mobile app for kindergartens, and our payment functionality has stopped working. We need an experienced developer to audit the app, find the root cause, and get payments working reliably again. About the app: Our product is a React Native / React Native Mobile Expo cross-platform Smartphone app covering both iOS Development and Android App Development, built with JavaScript and TypeScript, with Redux for state management. The backend runs on Node.js with MongoDB as the database, versioned in Git, and we use Firebase for auth and real-time data. It's a standard Mobile App Development / Native App Development stack — nothing exotic — but something in the payment integration has broken and we don't have the in-house expertise to diagnose it ourselves. What we need: A full technical audit of the payment flow — client-side (React Native, React, Redux) and server-side (Node.js, MongoDB, Git history of recent changes) Identify why transactions are failing (SDK version mismatch, API keys, webhook handling, Firebase rules, etc.) A clear written report of what's broken and why, following solid Software Development and Agile Software Development practice A fix, or a scoped quote for the fix, once the root cause is confirmed Nice to have: Native debugging in Swift (iOS) or Kotlin (Android) Familiarity with Flutter for future cross-platform work Front-end/web skills — HTML, HTML5, CSS, CSS 3, Bootstrap, Responsive Design — for our companion admin panel, which also touches billing Backend versatility in PHP, Laravel, Python, or jQuery Experience with WordPress and Angular for adjacent marketing/admin tooling A Desktop Application background for companion tools About us: We're a small, early-stage startup — this is a focused, practical engagement, not a big rebuild. We're looking for someone who can move fast, communicate clearly in plain English, and isn't afraid to dig into someone else's codebase. To apply, please include: A short note on how you'd approach diagnosing a broken payment flow in a React Native app Relevant experience auditing or fixing Mobile App Development payment integrations Your availability to start this week