After further testing, it seemed as soon as it uses my cellular data service, it got disconnected. When I turned off cellular data, I now am able to use the WiFi and get ATT's text message. Im trying to reproduce this.

Going to chime in that I have the same problem. I do notice when WiFi is off, it does get connected to the cellular network once in a while. It is now sitting in the living room with only cellular and I will not be willing to move it somewhere and use my Macbook/iPad to respond to text. (This is super frustrating esp when we depend on text messages for two authentication factor). Time to reconsider the implementation of two authentication factor with text message with this release. One quick question for you guys (and hoping Apple is reading and try to reproduce this). Are you using the eSIM or the physical sim card? mines' eSIM. (ATT) I did call Apple and opened a ticket... few days before my AppleCare expired! My iPhone is iPhone XS Max, 256 GB.