`whereHas()` multiple connections

 1// User Model
 2class User extends Model
 3{
 4    protected $connection = 'conn_1';
 5 
 6    public function posts()
 7    {
 8        return $this->hasMany(Post::class);
 9    }
10}
11 
12// Post Model
13class Post extends Model
14{
15    protected $connection = 'conn_2';
16 
17    public function user()
18    {
19        return $this->belongsTo(User::class, 'user_id');
20    }
21}
22 
23// wherehas()
24$posts = Post::whereHas('user', function ($query) use ($request) {
25      $query->from('db_name_conn_1.users')->where(...);
26  })->get();

Tip given by @adityaricki