For those with issues some to ideas are Disable and Enable RCS, clear data from Mobile Networks and Samsung Messages ,Reset the Network (of course may have been tried already).
See some more information here https://mobileinternist.com/rcs-not-working-samsung-messages
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 4.0,Android 12 Official Version.