Cellular Data Usage When Cellular Data is Turned Off?

I have an iphone 12 Pro on 14.5.1. I noticed that my cellular data usage was way above normal. Checked and saw that my phone was recording data usage when I was in my apartment on wireless. Wireless is working fine for other devices. So, yesterday I turned off all celluar data. This morning I see that the phone used cellular data at 1:15 am while I was asleep. Any ideas on what could be doing this and how to fix it?