How to fix: iMessage Unavailable: Turn on cellular data or use Wi-Fi to set up iMessage
Ok, so there's at least one solution:


Establish a Wi-Fi connection using Settings > Wi-Fi (switch on, etc.).
Log-in the iphone using AppleID when prompted.
Disconnect cellular using Settings > Cellular > Cellular Data (switch to OFF).
Switch iMessage on (Facetime, etc.) in Settings > Messages > iMessage.
Switch cellular data on by reversing step 3.
Switch Wi-Fi off if so inclined by reversing step 1.


That did it!

Also? Screw Apple! Talk about cut-rate functionality…