Solution 1: Turn off Do Not Disturb
If you are not getting notifications on iPhone, then first you should check if the "Do Not Disturb" option is turned on. As long as this option is activated, you will not receive any sort of notifications on your iPhone.
1. Look for the half-moon shaped icon on your status bar
2. If that icon is visible on the status bar then the "Do Not Disturb" option is activated on your iPhone
3. Now, go to Settings and turn off "Do Not Disturb"
Solution 2: Allow Notifications for Omni
Now, if you are not getting any notification for Omni, then,
1. Go to Settings
2. Tap on 'Omni'
3. Tap on 'Notifications'
4. Ensure that 'Allow Notifications' is always on (green)
Solution 3: Turn On Always Show Previews
1. Go to Settings
2. Tap on 'Notifications'
3. Tap on 'Show Previews'
4. Ensure that it is 'Always'
Solution 3: Reset Network Settings
If iOS 12 does not show notifications, you can try to fix it by resetting network settings.
1. Go to Settings on your iPhone
2. Tap "General" and scroll down to select "Reset"
3. Tap on Reset Network Settings and enter your passcode if prompted