@leek: If you're using the Samsung Messages app, can you try opening the app > Tap the 3 dots in the top right > Settings > Chat Settings > Rich Communication Settings > Toggle the switch off > Select Default Messaging Type > Text or Multimedia Messages, to see if this helps?
Are you sure RCS messaging is working on both devices? I had a situation when I could not set up RCS messaging on my S21 Ultra and left it on in the hope it would register with the server. While in this situation, my wife who had messages active on her phone had to turn messages off to send me an SMS otherwise nothing came through. Now my Ultra is registered and I no longer have this problem.