iPhone 13 Pro can't send or receive SMS

Hello,

Since 11/15/21 I have been unable to send or receive any SMS messages on my iPhone 13 Pro. When I try, it says "Not Delivered." When I am sent an SMS, it doesn't show up at all (I had my friend who has an Android send me a text while she was next to me.) This issue happens no matter where I am (at home, work, anywhere.) Everything else on the phone works fine. My data works, and I can send and receive calls from iPhones or Androids. The issue seemed to start when I switched my plan from Unlimited Extra to Unlimited Elite. For troubleshooting, I have tried the following so far:

1. Had my SIM card changed

2. Made sure AT&T has the right IMEI on file for my device

3. Reset my network settings multiple times

4. Made sure my iOS software is up to date

5. Made sure my messaging settings are correct (turned on send as SMS, etc.)

6. Called tech support for over an hour and they cannot figure it out!

If anyone else has had this issue, what fixed it? At this point I honestly am considering changing providers, because I need to be able to send and receive text messages from more than just other iPhones. It seems as though AT&T cannot fix the issue. I have been unable to login to some of my accounts as well because two-factor authentication uses SMS.  

Thank you.