@Cameron04Check you dont have a feature called WiFi assist on, which will use mobile data when it detects a drop in WiFi.
Also check for apps that will use Mobile Data and override the Phone settings.
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iPhone 16 Pro Max - o2 and Spusu
Xperia 1V - Spusu
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This is not customer services and we dont have access to your account
I do not work for o2 or any VMo2 /Telefonica/Liberty Global Company