On an Android device, download one of the apps calling itself WiFi Analyzer, and familiarise yourself with the WiFi environment you are operating in, both yours and your neighbours’, as shown by the various displays on the app.
Then, when the trouble starts next Friday, look at it again, note any significant differences, especially somebody hogging the WiFi channels your router is on, and report back.
Set a Payment PIN on your account so that no-one but you can buy memberships on it. Check your bank accounts monthly for any other unexpected payments to Now. That way you can at least nip them in the bud, while you and Now figure out whose fault they are.