Webview app with FCM

We need a relatively simple app built around a webview and FCM. When opened, the app would run thru an authentication procedure including a QR code and match this against a server. When this is done, it would download content from an URL and show this in a webview. Via FCM, it would receive data payloads for notification scheduling and simple settings changes that needs to be done in the app. This needs to work both when app is in foreground and background. For both iOS and Android, it should be able to run on as old OS-versions as possible so it can be used on older devices.