iMessage works on Mac, but not on either iPhone

All of the other discussions of this topic are several years old, and with the multiple updates I figured I'd ask again.

I'm trying to text a friend who has an iPhone. I texted him once from my iPhone and it worked. I tried another text and now it won't go out. All over wifi - we have cellular turned off because we are out of the US. However, I can iMessage him on my Mac and his works. Why?

BTW....all updated on latest everything on everything.