The "No SIM Card" started on my iPhone 11 Pro Max a couple months ago on the last version of iOS 13. There was little improvement with iOS 14.0 and again with iOS 14.1. I have AT&T cell service. Last night the cell service dropped during the night while the phone was being charged and not moving around. My workaround is to perform a Settings>General>Reset Network Settings. I then have to also reconnect to the home wifi network. This seems to work for a few days.
TooTallRon
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Portland, OR, United States