I don't want to use iMessage on an iOS 11 phone. How do I turn it off? How do I turn it off?

When I sent to Settings, Messages and turned off iMessages.  But then I am unable to SMS people with iPhones.

Error dialog pops up:

iMessage needs to be enabled to send this message.

I don't want to fix iMessage.  I don't want to use it.  I have unlimited SMS so don't see the point.  Is this possible?
Turn off iMessage by going to Settings>Messages and switching off iMessage. 

Then make sure "MMS Messagging" is turned on.

For more information check here:


  Deregister iMessage on your iPhone
  
  If you still have your iPhone, you can deregister iMessage with these steps:
  
  
  If you transferred your SIM card from your iPhone to a non-Apple phone, put it back in your iPhone.
  Make sure that you're connected to your cellular data network.
  Tap Settings > Messages and turn off iMessage.