Unable to send images and videos over iMessage - Message Failed to Send

I've been unable to send images and photos with iMessage. The delivery is attempted but it slows and I receive a Not Delivered warning and a notification Message Failed to Send. I've had no luck solving this problem with any of these steps:

  • Restarting the iPhone
  • Turning iMessage off and back on. And tried turning iMessage off, restarting the phone, and turning iMessage back on. You can do this from Settings -> Messages, and then toggling the iMessage toggle off and on.
  • Verified iMessage service was healthy at https://www.apple.com/support/systemstatus/
  • Tried on WiFi and over cellular.
  • Tried sending to individuals and group messages
  • Tried sending a photo to myself
  • Tried signing out of my iMessage account. Do this by going to Settings -> Messages -> click Send & Receive -> click Apple ID -> click Sign Out
  • Tried to de-register from iMessage at https://selfsolve.apple.com/deregister-imessage/
  • Signed out and in of iCloud