I think I found the fix! Here is what worked to restore service and stop the daily intermittent SIM failure errors after the 14.7.1 update to my XS and my wife’s 12. We are using eSIMs on AT&T. I removed our plans using Settings>General>Reset>Remove all cellular plans. Then I added them back using the QR codes on our carrier’s eSIM setup cards. This appears to have completely resolved our problem.
Before I tried this we had driven ourselves crazy rebooting, erasing cache and network settings, turning airplane mode on and off, wifi on and off, near and away from our wifi network, gotten a new physical SIM card for my wife’s phone, turned roaming and wifi calling on and off. Nothing worked until I removed all cellular plans.