After applying a system update, network-related functions such as iMessage could stop working due to network configuration issues. Some upgrades have a tendency to overwrite current settings, resulting in mistakes or conflicts. Resetting your iPhone's network settings might help clear this up. Your existing network settings will be erased, and the default network choices and configuration will be restored. Here's how you do it:

1. From the Home screen, choose Settings.
2. Select General.
3. Reset may be found by scrolling down and tapping it.
4. From the list of choices, choose Reset Network Settings.
5. To continue, enter your device passcode if requested.
6. Then, on your iPhone 7 Plus, press the option to approve network settings reset.