iPhone 6 using data when idle and on WiFi

I recently downgraded from an old unlimited plan to a 3Gig shared plan to save $40 a month. I also got an app that monitors cellular data usage and tallies it in 30 minute intervals. I plug my phone in overnight every night, and I have WiFi in my house. We also have the MicroCell as we don’t get very good cellular coverage. Since switching, I have noticed that every morning, usually during the same 30 minute interval, my phone uses about ~50megs of cellular data. This is while it is on WiFi and not even being used, I am not even up yet. If it uses 50 megs a day on both of our phones, that will eat up exactly 100% of our monthly data allotment. I have everything I can set to only use WiFi. I have been re-setting the phones cellular data stats each day to try to find the culprit. System services only shows about 400k every 24 hours, and none of the apps show mush usage. What gets me most of all is that I am on WiFi while this happens. At this rate, I will sue my entire plane without actually using any data myself. What could be using the cellular data?