App usage shows 'Wi-fi' is using all the data

[Android Central Question]

my SIM data usage has suddenly increased - from less than 500 Mb in a month it went to 2 Gb in one day and it's continuing.

Checking the App usage analysis on the phone, it shows 'Wi-fi' as the culprit.
How can Wi-Fi use my phone data I thought Wi-fi means you are using internet, not the cell phone network.
How can I stop this or at least minimise it ?

Phone is a Samsung S5 dual SIM card , providers is Three for the UK card and Orange for the French card. Problem arose with both suppliers and in 2 different countries and is continuing - I have had to stop the data complety to avoid the surcharges from Three and Orange.

Thanks !

 

[hallux]

Where are you seeing the WiFi data usage? On my phone there are 2 sections showing data usage - one shows how much data my phone used while on WiFi and the other shows data used while on mobile data. Are you saying that within the listing for mobile data there's an entry saying WiFi used data?

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