How to fix: iMessage Unavailable: Turn on cellular data or use Wi-Fi to set up iMessage My iPhone 5 suddenly ceased using iMessages. The phone is unlocked, has ios 9.3.2 installed, I have very few apps installed, and no new apps installed recently. 

When I go to Settings > Messages > iMessage and try to toggle, I get the following:


  iMessage Unavailable
  Turn on cellular data or use Wi-Fi to set up iMessage.


However, cellular data is enabled in Settings > Cellular > Cellular Data. Cellular data seems to be working fine in Safari, apps, etc. I get the same behavior with LTE or 4G cellular.

I have tried all of the following based on hunches and information gleaned from non-stackexchange sites:


Restarting Messages
Restarting the phone
Resetting network settings
Resetting the phone's passcode
Resetting the AppleID account password associated with this phone
Erasing all content and settings from the phone, and then restoring from backup
Ejecting the SIM card, and replacing it.


How to get this basic functionality to work again?
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…