I have had this issue on my 14 Pro Max, running on 17.0.3, where my network was completely knocked out for over two hours one day while I was on cellular. It was 'No Service', and multiple restarts, aeroplane mode switch, network settings reset DID NOT resolve this issue. I had two sim cards on my device coincidentally, one eSim one physical, and neither was working, so it was not an issue with the carrier settings as well. I tried enabling/disable both sim cards, nothing changed. It fixed itself two hours later.
Yesterday while I was on cellular once again, it was showing me bars on my cellular connection, but there were no calls coming in. Any body calling me was getting a 'Busy' message and then disconnected. Outgoing calls simply showed 'call failed'. This was fixed after an aeroplane mode switch.
Given the price of this device, and on a country like India where it cost me 1800$ for this device, this is absolutely unacceptable. I should not have to worry about why this is happening, software or what not. I shall wait for another update and monitor the connection in the time being. If it is not fixed, I'm going to have to switch to another manufacturer. I cannot accept the fact that a device which is shooting ProRes video cannot handle cellular connectivity.