hello Joe,
start by checking the radio subsystem: press *#06# on the keypad. Write down what appears on the screen. Then go to settings - >> general - >> this device scroll down. Find the position of the firmware modem and imei. It will be important what these numbers look like. This way you will confirm whether the radio system works in the phone or it has been previously damaged (what happens sometimes). y speaking, when the phone asks you to update the operator's data, there is a suspicion that the communication system is damaged. In 6s, the repair is rather standard and quick. It shouldn't be expensive anymore (it's 2016 phone).
crwdns2934105:0crwdne2934105:0
crwdns2934113:0crwdne2934113:0
crwdns2915270:0crwdne2915270:0
crwdns2889612:0crwdne2889612:0
1