update iOS fails on wifi (iPhone 7). need ATT network connection

hey you experts,

I have 2 phones. an android and an iPhone. the IP is a 7.0 and the iOS is a little outdated. I get a message saying I need to update the OS before I can make calls or send texts. however, the AT&T cell plan that I just purchased results in a triangle with an (!) exclamation point in it, in the upper left hand corner of the phone screen. so the service is unavailable. the agent told me this is a documented issue with iPhone 7 versions. I have attached 2 screenshots that are relevant. Can someone help me pinpoint the problem? I have my phone service switched at US Cellular from my android to my iPhone all the time and the iPhone has never had this problem. It has always worked perfectly. The only difference now is that the ATT agent took the old US Cellular SIM card out and put a new one in at his store. Does that even matter? thanks!

Adam

""installation_ready"" image.....

""no_service_no_verify_update"" image....