New SIM came with Samsung Galaxy S21 Ultra 5G 512 GB no mobile network (stuck on "connecting")

I received my new S21 Ultra 5G 512 GB on Aug 25th. I attempted to activate the new SIM card that came with it online at att.com/activate, and it failed. I tried this several times over the course of the next few days, and finally gave up. I took the old SIM from my old phone (S9+) and put it into my new S21 Ultra. This worked. After my business trip was finished, I tried to activate my new SIM again - same error as before. I finally decided to get assistance via chat on Sep 14th. I worked with Mike first, who activated my SIM so that it would at least make and receive phone calls. After I lost contact with Mike, I got Evelyn who worked diligently to try and address my issue of not having any mobile network (data) connectivity. After repeating one of the earlier steps of resetting my phone's network settings, Evelyn got disconnected from the chat (on my PC). Her supervisor (Aaron) called me to apologize and to continue working the issue. He asked me to reboot my phone, which I was talking to him on. He said he would wait 5 minutes and then call be back. When my phone rebooted , it seemed to reset its network settings a couple of times on its own. This must have prevented Aaron from being able to call me back. We have spent close to, if not over 3 hours attempting to get the mobile network connected. This is an almost $1400 phone, and it will not work unless I am on my wifi. There is no email address listed for support on the at&t wireless website, and I have sat in the chat queue for over 30 minutes total since yesterday. Please help me get this issue resolved quickly so that I can enjoy my new Samsung flagship phone and all its features.

Thank you,

Joe Root