@belle22 is this happening on Data and Wifi. Firstly an idea to go to Apps-Messages-Storage and clear the Data and cache? if not a solution Have you restarted the Device and tried reinserting the sim card (or spare one if you have it). In the end may need to contact your Network as others have said.
I do not work for Samsung or make Samsung Products but provide independent advice and valuable contributions.
My Device- S21 Ultra 5G ( SM-998B/DS ) CSC= BTU , 12GB/256GB
One Ui 5.0,Android 13 .