same happened with me too, since updating from iOS 13.x.x to iOS 14.1 when my iPhone XS Max signals low and gone then showing up No Service, no way to get the cellular network back, only I can do is force restarting the iPhone then it’s work again, very frustrating issues! I joined and wrote this comment is try to let Apple knows there’s more case with the problems. Hoping Apple can pay attention and fix please!

Dear All, I had same problem as before, 4G dropped to 3G and got 1 bar then No Service, only way to get service back only is force restart or turn off the phone and put back on or switch to Airplane mode 5 minutes, etc. All happened on iOS 14.1 and 14.2. After iOS 14.3 came out and followed other people methods with iTunes back up my phone then using DFU mode to restore my iPhone with iOS 14.3 and restore from back up plus manually choosing carrier not on automatic network mode, now my iPhone XS Max seems ok now for a week and half already.