I appreciate it's important to keep connected @Fransan
As a process of elimination can you try your sim card in another phone and another sim card in your phone to help rule out the phone or Sim Card at issue.
A Samsung Experience Store or Samsung Service Centre can run a diagnostic test.
I wish you all the best with this situation.
Current Phone ~
Samsung Note 20 Ultra 5G SM-9820 _256Gb SnapDragon 865+
Samsung One Ui 2.5/ Android 10.