Someone already have AW with Cellular can try below by force a carrier software update by resetting network settings, if work pls share here.

https://www.imore.com/how-roam-your-apple-w...foreign-country

A visit to the Apple Store didn't initially yield success, though a smart comment from one of the specialists led me to force a carrier software update by resetting my network settings; after running the update, the Watch app immediately offered me the option for setting up the device.

Once I pressed a few buttons and confirmed my emergency address, we were ready to go: I toggled my iPhone 8 Plus into Airplane mode and put the Seriess 3 GPS + Cellular model to work.