@Shepy: Before you proceed with the troubleshooting steps below, please reach out to your mobile network provider directly, to enquire about any known issues with the service in your area.
If you're having trouble sending text messages, then please try enabling Mobile Data by swiping from the top of the screen to the bottom > Mobile Data, as this is required to send and receive messages that exceed the SMS character limit, or contain images/emojis.
Now, go back to Messages > 3 Dots > Settings > More Settings > Text Messages, and make sure that a number is populated in the Message Centre section. If you can't see a number, contact your network directly to request the number, and manually enter it here.
To further troubleshoot the issue please open the Messages app > Tap the 3 dots in the top right > Settings > Chat Settings > Select Default Messaging Type, and set this to 'Text or Multimedia Messages'. Now, head to Settings > Connections > SIM Card Manager, and ensure that your preferred SIM is assigned to Messages. I also recommend restarting your device to address any temporary software issues that might be a contributing factor.
