Hey there
Sorry to hear of your issues. There could be a number of reasons for this but let's try a couple first:
1. Please perform a Reset Network Settings on your device. You won't lose data but you will have to re-enter your WiFi passwords
2. Where did you buy your device? Please locate the IMEI (you may have two) and go to one of the free IMEI check websites to make sure the device hasn't been network locked. (This may happen if the device was bought second hand and the seller claimed on insurance for example).
3. Have you tried a Factory Data Reset?
I am not employed by Samsung nor do I receive any remuneration as a Samsung Members Star.
My device is an S25 Ultra.