Cannot sync Apple Watch series 3 to cellular network

I received the Apple Watch series 3 as a birthday present and it was originally purchased on a different wireless account than the one my phone is on. The line was moved to my account and the watch was reset. I still cannot sync the watch using the number sync feature. It will allow me to sync the watch via Bluetooth, but when trying to activate the number sync, the phone routes me through the my At&T app and I get all the way to the numbersync section but it never sends a code to the watch. When I go to the cellular section on the watch it says No Sim under plan and No Connection under Status. Please help!