Could be various apps, updating in background; suggest you turn off MOBILE DATA on your phone.

Hi ​@Reeyapc,

I’m sorry to hear your data is running out faster than expected. I can understand how frustrating that must be, especially when you’re mainly using WiFi at home.
 

This can sometimes happen if mobile data stays active in the background, or if certain apps switch between WiFi and mobile data without it being obvious.
 

To help with this, please try:

Turn Mobile Data OFF when you are connected to WiFi (this is the most effective step)
Go to Settings > Data Usage and check which apps are using the most data
Disable background data for high-usage apps if needed
Turn off WiFi Assist / Adaptive connectivity (if your phone has it)
Make sure apps and system updates are set to WiFi only

Lamiya