So, i encountered a problem with my s21 ultra exynos version when it comes to its samsung messages, on some certain contacts i cannot send them messages unless the chat messages icon showing in the setting is off, i do this everytime because once i go back to home screen and then go to messaging app again, it automatically activates the "chat messages feature" again and i cannot send messages to them unless i turn it off..its getting annoying specifically when i thought i send the message at sometimes.
Turn off RCS fully if your contacts aren't utilizing it. RCS/Chat needs an internet connection for you to be able to send them messages. It will fail if your data is off. If Chat is important to you, I suggest you migrate to Google Messages as it is more stable in sending RCS. If not, again, turn it off in Samsung Messages fully. 
