The teaser will probably come out after TLJ home release is available, to maximise the advertising space for each product.
Umm, that makes no sense, so the teaser for Han Solo will come out in April 2018, when TLJ is released on home video, and the movie comes out in May 2018. That makes absolutely no sense.
I’m willing to wager it’s released with TLJ in the theaters in December.