Your phone can use mobile data while WiFi is connected if a connection-assist feature steps in, or an app is allowed to prefer cellular. First check whether the mobile-data counter is actually increasing during that session. Then disable automatic assistance or turn mobile data off for a short WiFi-only test. A WiFi symbol alone does not prove that every app is avoiding cellular.
This guide covers iPhone, Google Pixel and Samsung Galaxy settings checked on October 22, 2026. Menus vary by software and carrier. It is based on current manufacturer documentation, not a measured battery or data-use test.
Check what is using data before changing settings
A large total does not show when the data was used. It might include yesterday’s commute, a session before you joined WiFi, or a different billing period. Take a screenshot of the total, its date range and the largest apps before resetting anything. With two SIMs, check the line selected for data.
| What you notice | What to check next |
|---|---|
| The total is high, but does not change during your WiFi session | Compare the counter’s dates with the carrier’s billing period before blaming WiFi. |
| Data use appears near the backyard, driveway or a weak room | Check connection assistance, then repeat the WiFi-only test beside the router. |
| Only one Galaxy app keeps using mobile data | Inspect that app’s Allowed networks setting, not just the WiFi connection. |
| Websites stop loading as soon as mobile data is off | Check the WiFi network’s internet access or public-network sign-in. |
Use the phone’s breakdown to find a likely app, and your carrier’s record to investigate a charge. They are not interchangeable meters. Avoid running a video or speed test just to check for unwanted usage; a couple of ordinary new webpages is enough for the connection test below.
iPhone: check Connectivity Assist, not just Wi-Fi Assist
Apple’s current feature is Connectivity Assist. It can use cellular in addition to WiFi when WiFi is slow or unusable. Open Settings > Wi-Fi, scroll down and turn Connectivity Assist off if avoiding cellular use matters more than keeping a weak connection working.
Apple says this replaces Wi-Fi Assist, which was previously under Cellular. After a software update, it inherits the old feature’s enabled or disabled state. Older instructions pointing everyone to the bottom of Cellular therefore may not match your phone. Apple’s Connectivity Assist instructions also explain the option to disable it for a particular network.
Turning assistance off does not fix the network. At the edge of coverage, a page may stop loading instead of using cellular. If you prefer that trade-off on a limited plan, leave assistance off and address the weak WiFi separately.
Pixel and Galaxy: check automatic switching and app exceptions
On a supported Pixel, open Settings > Network & internet > Adaptive connectivity and check Auto-switch to mobile network. Google says it can switch when WiFi has no internet or the phone’s local WiFi connection is weak. Turn that control off if you do not want that fallback. See Google’s Pixel WiFi connection guide for the current setting.
On Galaxy, open Settings > Connections > Wi-Fi > More options > Intelligent Wi-Fi. Check Switch to mobile data. Samsung documents this as a switch away from unstable WiFi, with a return when the signal is strong. Availability depends on model and carrier; it needs active SIM service. Samsung’s connection options guide describes the control.
Also check Settings > Connections > Data usage > Allowed networks for apps on Galaxy. An app can be assigned mobile data rather than WiFi. Change an unwanted mobile-only choice to WiFi only, or the available-network option that suits you. This is a separate check from automatic switching.
If a named control is missing, search the normal Settings app or use your model’s manual. Do not enable Developer options, change hidden radio settings or delete an eSIM to solve this billing question.
Run a short WiFi-only check
- Choose a convenient moment, connect to the intended WiFi network and note its name. Close any ongoing download or streaming session.
- Turn mobile data off, leaving WiFi on. On iPhone, use Settings > Cellular > Cellular Data. On Pixel, use Settings > Network & internet > SIMs > Mobile data. On Galaxy, use Settings > Connections > Data usage > Mobile data.
- Open two new, ordinary webpages. Do not judge the result from a previously loaded page, saved music or an offline map.
- If they fail, try beside the router and check another device on the same network. If both devices fail, investigate the network or provider. If only this phone fails, compare it on another known-working WiFi network.
- Restore mobile data afterward if you need it away from WiFi. Leaving it off means apps lose their cellular internet fallback.
This separates “WiFi is connected” from “WiFi can reach the internet.” It is a suggested check, not a guarantee about future charges or a laboratory measurement of background traffic. There is no need to disable the entire SIM or use airplane mode for this test.
At a hotel or airport, look for a network sign-in notification or open a new browser window. Read the network’s terms before accepting them. At home, if other devices work normally, follow our WiFi troubleshooting guide for one affected device rather than resetting the whole household’s network.
Find the app and compare the right billing dates
iPhone: Open Settings > Cellular and scroll through the apps. You can disable cellular access for an individual app there. Preserve the existing totals before using Reset Statistics, then note the reset date for future comparisons. Apple also shows usage for the selected data number on Dual SIM. Its cellular data usage guide recommends the carrier as the most accurate reference for a current billing period.
Pixel: Open Settings > Network & internet > SIMs > App data usage. Choose the period above the graph and look at the apps below it. Google’s mobile data management instructions explain warnings and limits, and caution that the phone and carrier may measure differently.
Galaxy: Open Settings > Connections > Data usage > Mobile data usage. Match the billing cycle to your plan before comparing totals. Samsung’s Galaxy data usage guide covers the app breakdown, warnings and limits. Do not confuse its separate WiFi data total with cellular usage.
For a personal check, write down an app’s counter before and after a brief session with the same settings. A change is a clue, not an exact measurement of one action: background work can overlap. A carrier entry appearing later also does not, by itself, prove that the transfer happened while you were on WiFi.
When to stop changing phone settings
If your WiFi-only check works and the cellular counter stays unchanged during repeated sessions, keep your preferred assistance setting and monitor the matching billing period. Do not start a factory reset because an old total is still visible.
If WiFi works only beside the router, investigate coverage. If several devices lose internet together, contact the broadband provider. If the phone works on other networks, focus on the affected network rather than buying a new phone or a larger mobile plan.
If charges still look wrong, give the carrier the affected line, billing dates and your saved counters. Ask which allowance or service generated the charge. If your problem is choosing a cellular mode rather than unexpected usage on WiFi, our iPhone 5G Auto versus 5G On comparison addresses that separate decision.
