`whereHas()` multiple connections

// User Model
class User extends Model
{
    protected $connection = 'conn_1';
 
    public function posts()
    {
        return $this->hasMany(Post::class);
    }
}
 
// Post Model
class Post extends Model
{
    protected $connection = 'conn_2';
 
    public function user()
    {
        return $this->belongsTo(User::class, 'user_id');
    }
}
 
// wherehas()
$posts = Post::whereHas('user', function ($query) use ($request) {
      $query->from('db_name_conn_1.users')->where(...);
  })->get();

Tip given by @adityaricki