iMessage doesn't work

Hi, I have this friend and a few days ago our imessaging stopped working, and it switched to SMS. We both only have SMS with each other. Both of us have iPhone 7+, and only 1 number in our contacts for each other. We have tried rebooting our phones, and deleting our messages and starting a new conversation. Is there anyway to fix this and make our messages go back to iMessages

and not SMS?

[Re-Titled by Moderator]