Try the following solutions and check if your problem of iCloud backup greyed out is solved or not:
Check Restriction Settings of your iPhone
- Open Settings.
- Go to General > Restrictions > Accounts.
- Set this to “Allow Changes” if it is not yet allowed.
Check Airplane Mode Is Turned Off.
Update your iPhone to latest iOS version.
Reset Network Settings. Open Settings App. Go to General > Reset > Reset Network Settings.
Make Sure That You Have Enough iCloud Backup Space. If you do not have enough storage space, then you may encounter with iCloud backup greyed out issue on your iPhone.
I hope by following the above solutions might have helped you in fixing your issue of iCloud backup option is greyed out in iPhone 8 Plus running on iOS 12