Make sure that you have things set to update only over Wifi - you may think you are in Wifi range, but if the Wifi drops out, then it will eat data.
You can also set the device to limit mobile data usage.
Get a Wifi analyser app and check signal / congestion etc.
https://play.google.com/store/apps/d....wifi.analyzer - the one I use ... has ads.
NB. If ads update over mobile data, that can drip away - especially annoying if you have day rate data - one reason I deserted VM PAYG for Three's 1p/MB flat rate