Hi, I wonder if your phone is automatically connecting to WiFi without you knowing.
Make sure Mobile Data 'access data using mobile network' option in Settings, Network and Internet, Mobile, is switched on, but you say it already is.
In Settings, Network and Internet, tap WiFi, then WiFi Preferences and there is a 'connect to WiFi automatically' option. Try switching it off and make sure WiFi in Network and Internet is switched off in Settings also.
That might work if you are using Android.
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