Hi @Adam12345678910 ,
that is definetely not the behaviour expected from the S21 series phones and their updates. Have you tried to run a Harware test on the phone to check if there is any issue with it? You can do this from the Samsumg Members app> Get help> Under Diagnostics, select View tests> SIM card.
If the results display a message saying Action Required in red please do contact Support to get a repair arranged. You can find all their contact details HERE.
@Adam12345678910: I'm sorry to hear that your issue has persisted. If you head to Settings > Connections > SIM Card Manager, is your SIM highlighted at the top of the screen, and toggled on? If so, are your call and messages assigned to your preferred SIM?
If you have returned the phone to it's factory settings, and not introduced any third party apps following the reset, then I recommend reaching out to our Help Desk to arrange for the phone to be inspected by one of our engineers. I understand that this isn't ideal, and apologise for any inconvenience this might cause you.