This app was mentioned in 3 comments, with an average of 0.33 upvotes
perhaps with an app?
i dont know anything that can monitor realtime but i use data usage to moitor how many GB im using. link to data usage https://play.google.com/store/apps/details?id=com.sigterm&hl=en
It is against this subreddit rules, for recommendations for features.
You could use an app instead eg: -
https://play.google.com/store/apps/details?id=com.sigterm
^ Edit: - just tried above app, dark themed so difficult to use.
Taken from this page: - http://mashtips.com/data-usage-monitor-android-app/