Can I use a cellular Apple Watch with no iPhone

The SMS/MMS texting requires the use of the traditional voice cellular network.

The Apple Watch uses an LTE data only cellular radio, and a region specific one as well. This is so that the cellular radio chip is small, and does not consume a large amount of power (there is not much room for a big battery in the Apple Watch).

Cellular calling on the Apple Watch is via VoIP (Voice over the Internet) and only used cellular data.

Since SMS/MMS requires a different cellular radio chip, Apple relays all SMS/MMS texting via the larger more capable iPhone cellular radio, using the iPhone's much larger battery.

This is why SMS/MMS requires the iPhone to be turned on, able to access the Internet. Having FaceTime logged in on the iPhone is how the Apple Watch relays the SMS/MMS texts, since FaceTime has the infrastructure for keeping track of the iPhone and the Apple Watch.

I bought the most recent version (Apple Watch 6) and learned that the Apple Support response that you can use the Watch for SMS texting is not completely accurate. It is impossible to send or receive SMS text messages if your phone is turned off. Your iPhone must be turned on and connected to wifi or cellular. After 6 releases this is crazy ridiculous. I returned the watch. Waiting for real SMS texting support.

It wouldn't work,the apple cellular watch uses the number of the iphone it's paired too in order to make calls and send texts,which means you need an iphone with ios14 or higher.