Unable to connect cellular on my child's Apple Watch with AT&T

After no less than 24 hours in the store at ATT and on the phone, I'm at a loss of what is wrong with my child's watch. Last year we set up my first net iPhone 12 with three Apple Watches on the ATT and Family plan (my youngest A got my old SE, the oldest K got a new SE and I got a new 8). A broke her watch and ATT replaced the watch through the warranty with a nice new (and big) 9. I took my 8 and her 9 to ATT to have them activate the watch and switch the accounts so she would have the smaller screen 8 and I would take the 9.

The 9 successfully got on the cellular network. NO ONE can figure out why the 8 will not connect to cellular. On the ap- for A's watch it lists ATT for cellular, on the watch it says "no SIM no connection."

After multiple pain/unpair attempts, ATT suggested I replace the 8 (maybe it was fried from all the attempts). Apple kindly sent a new 8 in exchange. This one will still not connect.

ATT even went as far as to suggest that apple supplies the cell service to the watches (hello, I pay my ATT bill for the watches....) and they added apple to the phone call. Tech support at apple says it's an ATT problem but are trying their hardest to help. If I didn't owe ATT $800 back in bill credits, I would leave. They won't even comp the bill for A's cellular plan that can't be used. ATT customer support has been horrific and incompetent. They were suppose to have a "customer service supervisor" call up back in 48 hours.... that was last week. When we call and ask about this supervisor calling, they don't have a clue what I'm talking about. If you have made it this far, thanks for reading. I hope someone smarter than ATT can tell me why this watch that was connected to their network previously, will no longer connect.

**all devices have been updated

[Re-Titled by Moderator]