First I would update your phone's software. For my Samsung Galaxy S21 FE I've been having my phone say "Searching for service" on the lock screen and it will say not registered on a network when trying to make a call. No calls or texts but data works fine. What worked for me was going to Connections > Mobile networks > Network operators, unchecking select automatically and selecting home. I think it is a software bug with my phone that causes this because it seems to happen randomly and will resolve on it's own after a day or so otherwise.