Same issue here. iPhone XS worked just fine for the first three days. Then, I left home for a road trip 5 hours away and the LTE started cutting in and out. I did some hard resets and power downs, which worked. But this morning all that stopped working and the LTE would not come back. Phone was showing "SIM failure" a lot. I have an eSIM with ATT, so I thought maybe that was the issue. I reset the network settings, toggled airplane mode, cell lines on/off, etc. All the things already reported in here. Didn't work. Took the phone to AT&T. They said I was the 6th person who came in today with the same issue and advised me to go back to iOS 13. 🤦🏻‍♂️ I decided to get on the iOS 14 Beta instead, to see if that worked. Well, I downloaded the Beta program profile to my XS and my LTE immediately came back. Never updated to the iOS 14 Beta at all. LTE is working fine now, somehow. Not sure if this is the solution for everyone else, but it worked for me. Still running iOS 14.0, but the profile download for the Beta program seems to have cured my phone, at least.