iCloud Backup

Update: I've wrung this out and when I say I've wrung this out I mean I've wrung this out.

• Grayed out = restricted
• Only way to restrict it is via Configuration Profile (apart from it being a transient bug fault)
• Period

Go on, then. My compliments to you for bringing this to people's attention. 💪🏻

Thanks for the heads up and my compliments to you for the comprehensive screen shots. You're still not connected to Wi-Fi there, just cellular/mobile 4G and very low signal level at that. Here's the symbol that'll show up when you're connected to Wi-Fi:

First connect the iPhone to iTunes on your computer. Once it's connected, click on the Summary tab in the left-hand pane right under Settings. In the right-hand pane, click on Check for Update even it says it's up to date. It should detect and allow you to update the iPhone to second build of iOS 12.1.2. While you're there, under Backups, if This Computer is selected under Automatically Back Up, change that to iCloud!

Then, with reference to the following HT articles, connect the iPhone to Wi-Fi and try again.

• Connect to Wi-Fi on your iPhone, iPad, or iPod touch
• If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network

If still no worky, then maybe either the IP ports used by iCloud Backup are blocked from accessing the Internet by whatever network you're on or the function is restricted, possibly via a Configuration Profile installed on your iPhone especially if the iPhone is enterprise-owned and managed.

• Settings > Apple ID et c. > iCloud > STORAGE: how much of how much used?
• Settings > Apple ID et c. > iCloud > Manage Storage > Backups > BACKUPS/This iPhone > Backup Size? Next Backup Size?
• iOS version?
• Wi-Fi's good?

Settings > Apple ID et c. > iCloud > iCloud Backup > enable it

It's not connected to Wi-Fi in these last screen shots so, just curious, which build of iOS 12.1.2 are you running? 16C101 or 16C104? Settings > General > About > Version