Apple Watch Ultra not sending or receiving SMS text.

Hello, I have an Apple Watch Ultra 2. I left my iPhone at home, charging, and it is powered on connected to wifi. I thought I was able to send and receive SMS messages on my watch as long as 1. the watch is connected to wifi or cellular and 2. the iPhone is powered on and on cellular or wifi. Is that not the case?

My watch is connected to cellular and when I try to send a SMS, it shows a green bar at the top, about 90% or so, hangs there for a few minutes before saying "Message failed to deliver"

What am I doing wrong?

Thanks.