Can you as a process of elimination try your sim card in another phone just to make sure the sim hasn't gone faulty and check with any available network mast checkers to ensure its not your network at issue.
If your sim works fine in another phone and it's not your masts then try resetting your network connection in settings, reset and reset network connections.
If that does not help then return the phone for a new one and test it in the store with your sim before leaving if this wasn't sent by courier obviously.
A Samsung Experience Store or Samsung Service Centre can take a look.
I wish you all the best with this.
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 2.5/ Android 10.