SMS 'network is not ready' message

'network is not ready' message pops up when I try to send or reply to SMS messages. I'm using Google messages on a OnePlus Android phone. Any solutions please appreciated.

Is it RCS your using possibly
Did it stop working after Porting number

Thanks for you reply. I get the same message with RCS switched on or off. In fact with RCS on it says 'Status. not supported, no compatible SIM card detected' . I'm mystified.