Why does mobile data work in my browser but not in one app?

Not as the first step. Check whether the app can use cellular data, then compare the same action in the app on Wi-Fi and mobile data. That helps distinguish a per-app restriction from a failure that occurs under particular connection conditions. Record what happens before trying other steps, and check the app maker’s guidance before reinstalling.

In Settings, open Cellular and find the affected app in the app list. Check whether cellular data is enabled for it. iPhone allows cellular access to be turned off for individual apps; when it is off for an app, that app uses Wi-Fi only. The list also shows app data usage. [S1]

If access is on, test the same action once with Wi-Fi and once with Wi-Fi turned off and mobile data available. Change one thing at a time and note the result. If the app fails on both connections, or only one feature fails, record that too; a working browser does not establish the cause of another app’s problem.

It can be a difference in connection conditions. Apple describes Wi-Fi Assist switching to cellular when Wi-Fi is poor, using Safari as an example. It works only with foreground apps and does not activate for some third-party streaming or attachment-download apps; roaming and Low Data Mode also affect availability. [S2] This may explain why browser and app behavior differs, but it does not diagnose the specific app.

For a clear comparison, note whether Wi-Fi was on or off, what action you tried, the exact error, and whether the result was consistent. You could send the app’s support team a short report like this:

“App: [name, version if available]. Device: [model]. Approximate time: [time and time zone]. On Wi-Fi: [result]. On mobile data: [result]. Cellular access for this app: [on/off]. Error: [exact wording]. Steps tried: [list].”

Keep the report factual and leave out passwords, one-time codes, and other sensitive information. Check the app’s own help or status information if available.

Consider it only after checking per-app cellular access, comparing Wi-Fi with mobile data, and recording the error and affected feature. Follow the app maker’s guidance if available. Before reinstalling, make sure you can access any account details needed to set the app up again. If you are unsure whether locally stored information could be affected, ask the app’s support team first rather than assuming it is backed up.

The useful outcome of the checks is not necessarily a diagnosis: it is a clearer report of which connection, action, and conditions are associated with the failure.