I had similar issues with my iPhone 17 Pro and AT&T Mexico. It was never fixed so I ditched the device, replaced the e-sim with a physical one and went back to my iPhone Mini 13. I wouldn't buy another iPhone without a sim tray anymore.
A cellular issue affecting the iPhone 18 Pro Max on AT&T's US network can result in a complete loss of connectivity. Apple recommends two updates as a preventive measure. However, if the issue has already occurred, the company says the only solution is to replace the device.