Unexplained data usage

[Lee-the-Carer]

Due to circumstances I spend 99% of my time at home, so only use wi-fi and mobile data is always turned off. so usually have zero usage?
This last month the signal has been patchy with days of no service (contacted o2 and they say it's not an area problem) although if I do go away from home I get a signal.
So was surprised to find out that this month I had used 90% of my data allowance in the last 2 weeks?
is there a way to find out who is using my data because it's not me?

[MI5]

@Lee-the-Carer 

Check the data counter on your phone and if it shows zero usage, you'll need to contact O2.
All numbers are in this guide Guide: Coronavirus Community Help and Support 
When you call, you will be asked for the reason you are calling.
Stay silent for 15 seconds and you will be taken to the keypad options.
Press 2 for Everything Else
Press 5 for More Options
Press 7 for Anything Else which will put you though to someone.
You can also try saying "Upgrade" or "Fraud" to be connected to an advisor.
8.00am is the best time to call if you can.

[Lee-the-Carer]

Thanks for the advice,

not sure about the data counter as my phone doesn't separate current from lifetime? both show the same figure.
If it carries on I will call and hope to speak to someone, last time when I got through on my PC about having no service they said I had to use an activation code sent by text, which never arrived as I had no service!!

[MI5]

@Lee-the-Carer 

You can reset the data counter to keep an eye on it from now on.