iCloud pop up won't go away, no internet connection to fix it

My husband is out in the field, no internet connection, but his phone has the debilitating iCloud pop up which asks him to sign in and neither recognizes his Apple ID nor allows him to choose "Cancel." The only answers I have found to this issue on the internet are those that eventually come around to signing into an internet connection to either log in, delete iCloud, change settings, etc... The rub is that he is no where near any kind of wifi or land connection. He's tried turning it to airplane mode - still gets the pop up. He's turned it off and on - still gets the pop up. He's tried everything he can think of.

He will not be in town for another month but needs to use his phone for some of the basic apps like Alarm Clock and timer. But can't because of the pop up.

Any thoughts?

Thanks,