Just got my phone. in 15 days it used 2GB of my 4GB. My average usage with prior companies was 1.2GB a month. I am using WiFi, but I am also having a issue with the phone sucking data from our Wifi causing use to go over on Mediacom wifi. I have now turned off iCloud drive from cell data and also turned off iCloud back-up. what else could cause this?
There are several things which could have caused this. When first setting up your device, if you are not on wifi when doing so, the phone automatically downloading all of your apps would count against your cellular data.
Apple has this feature, not easily found on your phone called wifi-assist which will let the "phone" choose to use cellular data instead of wifi when you are connected to wifi if the "phone" thinks it is a better choice. Apple obviously thinks the phone can make that choice better than you can. You need to make sure this is disabled in your settings under cellular data and it will be located at the VERY BOTTOM of the list of all the apps which use data on your device. On my phone that is at the bottom of a VERY LONG list if you have a lot of apps on your device.
The settings on any app you have on your phone could have that app using background data to keep it up to date. A notorious app for using a LOT of data is Facebook. I am sure there are many others and you should check the setting on each of the apps on your device.