A certain reddit user brought Pixel 3 under the spotlight again. This post tells that Google Pixel 3 and 3XL users are facing the LTE connectivity issues on the AT&T network.
Users report about the issue on AT&T forum as well. They say, as soon as you disconnect from the WiFi, the device fails to connect to the cellular network despite being in a perfect coverage area.
The connectivity symbol on Status Bar shows an X and can only be brought back on the network by changing the network. Also, some also mention that the replacement devices are also having a similar issue.
The issue behind this? Some say it is the problem with Band 30 or some setting changes in Developer Options. Or it might be a change in APN settings? Well, no one knows, unless Google and AT&T post about it.
Source: Android Authority