Your SIM card is out of date. You need to get a new one from your carrier.
This solved my issue with no carrier signal after updating to IOS 14.2
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From the information provided to me by my carrier. With the IOS 14.2 update. The software interface was made for 5G spectrum. Therefore if you are using an old SIM card that does not support this. This will not work.
This old SIM card will work on any other iPhone/Android phone etc. or IOS less than 14.2 not based for 5G interface. Therefore some users can use their SIM card on an older iPhone and those that are updated up to IOS 14.1 But specifically not on the iPhones that have been updated to 14.2
Therefore, if you get a new SIM card, that is capable with the new interface of 14.2. Which can be provided from your carrier. (Whoever that is), would solve the issue of “no cellular data/service” The only issue is a new SIM card could cost some money, it’s a hassle to go and get it and even if it works. You might be stuck on 3G or LTE and not 5G depending on what spectrum is available to your iPhone i.e. iPhone X will not get 5G obviously.