My phone says SOS only even after I've tried restarting it

SOS means that your phone is working, but is not connected to your carrier’s network. It can only make emergency calls on other networks. You need to contact your carrier to resolve this. Here is guidance from Apple→If you see No Service, Searching, or SOS on your iPhone or iPad

Here’s a way to check up on your carrier→Down Detector - Crowdsourced Outage Check