Several iPhone 12-series device owners have taken to the Apple Support Community to complain that their new flagships are losing connectivity while using 5G networks. Many of them report that they are in the United States and are on the Verizon or AT&T networks, although one claims to be in Australia.
https://www.notebookcheck.net/iPhone-12s-on-AT-T-and-Verizon-are-losing-5G-signals-in-the-US.507575.0.html