Lumia 435 + Data Sense

[Windows Central Question]

I have a lumia 435 on 02 pay & go. I have had the phone for 2 days and it has used 200mb of data and now tells me i am 20mb over my limit. If i have no data left on my pay & go how can i go over.? I am also on wi-fi at my home so i thought it doesnt us available data it is free on wi-fi. i have changed settings advised on the internet but still no difference. On my data sense screen it says Mobile 20mb WiFi 1mb. The WiFi part never changes. HELP!!!!!!!