Apple Watch - cannot activate FirstNet Cellular Service on it

Support docs PDF

https://www.google.com/url?sa=t&source=web&rct=j&opi=89978449&url=https://www.firstnet.com/content/dam/firstnet/white-papers/firstnet-numbersync-apple-watch-setup.pdf&ved=2ahUKEwiw9b61y5OBAxWoq4kEHfBpD70QFnoECBoQAQ&usg=AOvVaw2nTfpOBvc--NfMc7EcLhv6

More 

https://www.firstnet.com/help/device-help/numbersync-for-wearables/numbersync-for-apple/numbersync-setup-for-apple.html

If you're still stuck, I would recommend either an AT&T store for assistance, call support, or a genius bar appointment

It's pretty unusual for the Apple watch to be so defective that it doesn't work with this process. But number sync is the worst program. It's terrible with Androids, usually works with Apple devices

The watch is on its way back to Apple for a repair.