Many iPhone X users are facing issue that the iPhone X is consuming too much cellular data. For some users reported that the phone is taking 3GB data in just 2 hours of data. Many customers reported this issue to Apple, but the customer care seems to be completely unaware of the issue. Most of the usage comes from System Services under the "Apple ID Services" heading. It was hundreds of MB a day while still connected to WiFi.
iPhone X is best flagship phones All-in-all, the phone is amazing. But such issue makes many iPhone X users regret their $1000 purchase. The users even tried to contact to service provider like AT&T and Verizon wireless to debug the issue but no updates are found.
One user reported the issue of reddit as quoted “Ever since, the data usage has been insane on this phone, to the point where I have contacted Apple and they have told me to turn my data usage off… Apple has given me no answers, other than the fact that the iPhone X has enhancements that might use cellular data regardless of a wifi connection.”
How to monitor data usage and fix iPhone X excess data usage:
Goto Cellular screen and turn off WiFi Assist, Data Roaming, iCloud Drive. After that, reset your stats and monitor which apps are hogging data and switch them off too if you don’t need them while out and about.
Appears that when GPS is being used and Cellular Data is enabled, Time and Location uses an excessive amount of Cellular Data.
Go to settings down to apps disable using cellular data on all the apps you don't need to use cellular data. Then they will just work on wifi.