Cellular connection going into SOS mode

The first thing I would do is run this to get a baseline: Apple Repair Assistant. This will at least give you a baseline of what to expect and include any possible HW failures. Watch for an eSIM issue - dead eSIM=dead cell radio.

After you run the Repair Assistant, I would try to reset the network settings. That said, radio failures are not common, but I have seen them more on the 11 and 12 Pro to a lesser degree than the other phones, which are infamous like the Intel 4G GSM only (AT&T/T-Mo locked exclusive variant) A1660 iPhone 7 and to a lesser degree, the CDMA Qualcomm iPhone 7.

BEWARE: If the cellular option greys out, the radio is likely dead, but you can at least save the data by doing a WiFi or PC backup. This is key, as the last option will ruin any chance. I would encourage you to use this window before going nuclear and potentially bricking the iPhone.

If, for some reason, it is already greyed out under Settings, there is another option you can try: Put the phone into recovery mode and then update the device there, as it will reload the radio firmware and potentially fix this.

CAUTION: BACKUP THE PHONE FIRST BEFORE DOING THIS! IF THE RADIO CHIP FAILS WITHOUT A BACKUP THE PHONE WILL LIKELY NEVER ACTIVATE AGAIN - AND YOUR DATA IS G O N E. Let me say it again: GONE.

I will say that most of the time, when the cell chip is dead with iPhones, it just takes newer iOS builds and the cell option remains dead. The problem is that you're dead in the water if it fails and doesn't take the update, and your phone is in a recovery mode loop