Apple Watch SE won't connect to cellular

 Family sharing is not the same thing as family set up. You need both to set up a stand alone watch.  

     You do not need or use numbersync.   Numbersync allows to watch to spoof your phone number for call text and data. You do not want this. 

Factory reset to the device and start over as below.  You may need assistance from AT&T if this adds an additional watch line to your plan.   Check your account. 

https://support.apple.com/en-us/HT211768

https://www.apple.com/family-sharing/

You may also need to contact AT&T about removing number sync from your line if you do not have a companion device of your own such as a tablet or your own Apple watch

If you are still unable to make the watch a standalone device with a Cellular Connection, contact Apple about a warranty replacement.  (AT&T is not permitted to warranty Apple devices, warranty is directly through Apple) 

To clarify, I do have my child as a member in Apple's family sharing.  I used the family set-up feature when setting up the apple watch. I do not have a wearable. When I purchased the watch, ATT created a separate line for the device.  When I get to the cellular set-up step, it usually errors out after the emergency number part of the process.

I have suggested it was a numbersync issue to ATT tech support, and I was told that numbersync is required. Is there a way I should phrase the request to remove numbersync. Do I ask that it gets removed from the wearable phone line? 

AT&T chat Reps don't know diddly Squat and are probably not in the United States and not even using AT&T.

Numbersync is only appropriate if you intend to use the watch as a companion device spoofing your phone number. With numbersync set up, It will not use the watch number, and will not act as an independent device for your child. 

If numbersync has been added to your phone line please contact AT&T and have them remove it. You will then need to factory reset the watch and set up with Apple's family set up as instructed in the link.   I'm a little surprised tech support made this mistake. They should know very well that Apple's family setup is not the same as using the watch as a companion device and therefore does not use number sync

This may seem like a dumb question, but was the watch factory reset and a cellular version? 

The GPS version of the Apple watch has no cellular radios and no e-sim.

Adding a line for the watch should be automatic during the setup process as a standalone device. That's why when you factory reset and then go through the process of setting the watch up with family setup it will create another $10 line on your account. Leading to the inevitable phone call to AT&T to cancel the previous line and request credits. 

If none of this puts you back on track, it would suggest to me there's something defective with the watch.  

You might also consider making an appointment with the Apple Genius Bar. They should certainly know how family set up works and can probably give you better directions than AT&T tech support. My faith and tech support is shaken by them suggesting that you use number sync which is completely wrong

Yes it is a cellular model. The watch has been factory reset numerous times and esim has been removed and readded/updated several times as well. The number and new line was created at the ATT store when I bought the device. They handed me the box with the watch in it and sent me on my way to set it up at home. 

I just received the watch back from Apple and they say that nothing is wrong with the watch.  I will try my luck with your suggestions. Tech support has been far less than helpful.

Hello, @nabig! We want to help get the Apple Watch connected to cellular data!

Thank you @formerlyknownas for your helpful information.

Based on the information you have provided, let’s meet in a Direct Message to further discuss how to get the watch cellular data connected. Please check your Direct Message Inbox (it's the chat icon next to the bell icon in the upper right corner of the Forums). 

In the meantime, the NumberSync does need to be unpaired for Family Setup.

We look forward to chatting with you soon!

Rachel, AT&T Community Specialist

I have exactly this issue. Please can you help me too. Thank you. 

Unfortunately ATT was unable to solve my issue. I am left with a cellular model that only works on wifi and I had to order my daughter a phone. I am extremely disappointed. I got caught in an endless loop of ATT blaming Apple and Apple blaming ATT.