Yes, in order to use your Apple Watch with a cellular data line, you typically need to have a SIM card or an eSIM (electronic SIM) provisioned for the watch.
Here's what you should consider and check:
1. **Physical SIM Card or eSIM**: Some cellular Apple Watch models support a physical SIM card, while others use an eSIM, which is a virtual SIM embedded within the watch. You may need to check the specific model of your Apple Watch to determine whether it requires a physical SIM card or uses an eSIM.
2. **Activation and Setup**: When you add cellular service to your Apple Watch through your carrier (in this case, AT&T), they should guide you through the process of setting up the cellular plan and activating the SIM or eSIM for your watch. This usually involves syncing the plan with your iPhone and the watch.
3. **SIM Activation Issues**: If you're encountering a "no SIM" message even after setting up the service with AT&T, there could be a few reasons for this issue:
- **SIM Provisioning Delay**: Sometimes, it might take a little time for the carrier to fully activate the SIM or eSIM for your watch. Try waiting for a while and then restarting your watch.
- **Incorrect Setup**: Double-check that you followed all the steps provided by AT&T for activating the service on your watch correctly. Sometimes, a minor mistake during setup can lead to connectivity issues.
- **Compatibility**: Ensure that your Apple Watch model is compatible with cellular service and has the necessary hardware for using a SIM or eSIM.
4. **Contact AT&T**: If you continue to experience issues, it's best to contact AT&T customer support. They can help troubleshoot the problem and ensure that your watch's cellular service is properly activated.
Remember that specific steps and requirements may vary based on your watch model, carrier policies, and any recent updates. It's important to follow the instructions provided by your carrier and, if necessary, seek assistance from their support team to get your Apple Watch's cellular service up and running successfully.