Practical guide · verified against the real thing
Mobile data not working: the diagnosis that finds which layer failed
In one line: Wi-Fi fine, data dead? The ladder from the toggle you checked twice to the APN reset almost nobody needs — and the four settings that silently eat data.
Wi-Fi works, the signal bars are full, and mobile data is dead. The failure lives in one of six layers, and the fastest diagnosis walks them shortest-first: the switch, the cap, the coverage, the carrier setting, the phone, and only then the exotic fixes.
The ladder, shortest first
1. The obvious switches, properly. Mobile data on (Quick Settings and Settings, then Network — the two toggles are genuinely separate on many phones), Airplane/Flight mode actually off, and if you're abroad, data roaming on — the setting that's off by default on most phones and silently kills data the moment you cross a border, including sometimes between networks at home. Roaming charges are a real thing to understand first: check your plan's roaming terms before flipping it abroad.
2. The cap you forgot. Data-limit features and carrier fair-use throttles produce the same symptom as total failure: apps time out, the speed-test crawls. Check the phone's data-usage screen for a set limit, and the carrier's app or balance line for the real remaining allowance. Unlimited plans still throttle after thresholds — timestamped usage logs settle "when did it slow down" arguments instantly.
3. Coverage, honestly tested. One bar in a basement is a coverage problem, not a phone problem — step outside, or check the carrier's coverage map for your exact location. 5G can also be the culprit in reverse: a phone clinging to a hopeless 5G connection does worse than a solid 4G one; forcing 4G (Settings, then SIM, then Preferred network type) is a legitimate fix in weak-5G areas, not a defeat.
4. The VPN and the APN. A VPN app with a dead server tunnels your data into a wall — turn it off for one test (this same suspect breaks Wi-Fi onboarding too, as the Wi-Fi diagnosis notes). If the VPN is innocent, the APN — the carrier's access-point settings — is the last software layer: Settings, then SIM, then Access Point Names, then "Reset to default". Wrong or corrupted APNs are rare, classically hit after switching carriers or SIM-swapping, and the reset fixes them in one tap.
5. The phone, rebooted and SIM-checked. The restart reflex exists for a reason — it re-registers the phone to the network, which resolves the "shows signal, delivers nothing" state after outages or long subway rides. Then reseat the SIM (power off first): oxidised contacts produce intermittent data exactly like this. Try the SIM in another phone, or another SIM in yours — one minute of swapping answers "phone or SIM" definitively.
6. The carrier. Worked the ladder, dead everywhere, no outage on the status page? Check for an outage page or community thread first (carriers have them), then call support with the evidence: SIM tested in another device, APN reset done, outages checked. That ticket gets fixed same-day; "it doesn't work" does not. And for the reverse situation — data fine, Wi-Fi broken — the same method lives in the Wi-Fi checklist.
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