Hello,
Just yesterday my phone started doing the same thing I tried everything I knew by resetting it and turning the phone off and back on taking out my sim but nothing worked. So I called my carrier to see if the towers was down or if they could fix the problem I was on the phone for over an hour with them. So, I ended up calling apple it turns out that some of the IPhone 7 that has failed on the main logic board. So they told they told me they would replace my phone free of charge. I would give apple a call as soon as possible so they can fix it. These are the model numbers that seem to be having this problem. I hoped this helped you.
Model number Country/Region
A1660, A1780 China
A1660 Hong Kong Macao U.S. (includes Puerto Rico and U.S. Virgin Islands)
A1779 Japan