Message not sent: Service not activated on network

I've recently changed phones (old and new both Samsung), and now I can't send photo messages on Samsung Messages and Google messages, I get a message that comes up saying [Message not sent: Service not activated on network]. I've tried numerous things, including requesting the Lebara settings text, and deactivating my phone number on the Google RCS site and restarting the phone. I've not got a clue what's causing this. Anyone got any advice?