If the issue is caused by network issues or the wrong network settings configuration on the iPhone, resetting the network settings should resolve the issue. Please keep in mind that resetting your network settings will also remove your current network settings, including Wi-Fi networks and passwords. So make a note of them before you begin. Once you're ready, follow these steps to reset your iPhone 8's network settings:

1. From the Home screen, select Settings.
2. Select General.
3. Scroll down to Reset and tap it.
4. Reset Network Settings should be selected.
5. Enter the device passcode if asked.
6. Tap to confirm that the network settings have been reset.
7. Once the reset is complete, your iPhone will reboot. Wait until it completely boots up before opening the Messages app and sending a text message.
8. Resetting network settings returns all data connections to their original or factory default settings.