Chosen Solution
I don’t think it’s a problem with the SIM reader. I tried a working Verizon one and it called fine when I turned on WiFi Calling, but it still showed Searching and then No Service. If it was a baseband IC chip problem, wouldn’t the Modem Firmware be unavailable? Earlier today, I turned off Airplane Mode for whatever reason since I always keep it on and the Verizon text actually showed up with about 3 bars and went back down to No Service within a minute; I’ve never seen it do that before.
No service/intermittent service is an internal problem that should be replaced by Apple. There could be a baseband chip issue - you may send the device to a micro soldering establishment.
Apple can fix it if you send it in or take it to a Apple store for repair. If it’s in warranty they’ll fix it for free, if not the price varies.
