I know there was a major AT&T outage at this time, but today is March 8 and I am on Verizon. I updated and had the same problem.
When you start the update it asks if you want to connect to WiFi or use cellular data. The problem is that the latest update, 17.4, updates your carrier services.
I said I didn't want to connect to WiFi and so after it "finished" I couldn't connect to my carrier. I connected to a WiFi hotspot, and the phone said it wanted to update my carrier services. I let it do that, waited, still SOS. So I restarted the phone and all is good.
It seems like for Verizon at least, you need to be on WiFi when updating to 17.4