Wordpress show posts from certain category on first page

Question

Id like to have on homepage on first page posts only from certain category and when user click to next page, he will see everything else.

I added this to my functions.php :

function my_home_category ($query) {
    if ( $query->is_home() && !($query->is_paged()) && $query->is_main_query() ) {
    $query->set( 'cat', '2');
    }
}
add_action ('pre_get_posts', 'my_home_category');

And it works. The only problem is, when I go to page 2 , I see posts that would normally be at page 2 (if I didnt have this function) and not the newest posts from all categories. So if earlier there were some posts from other categories on the first page, I cant see them now anywhere.

So before the function my blog was like:

Page1: A, B, C, B, A <br>
Page2: B, A, A, C, B <br>

(A means post from category A ...)

And I would like to have it like this:

Page1: B, B, B, B, B <br>
Page2: A, C, A, A, A <br>
Page3: C ... <br>

But what happened is:

Page1: B, B, B, B, B <br>
Page2: B, A, A, C, B <br>

Can you help me please ?

Answer 1

Modify your function something to:

function my_home_category ($query) {
    if ( $query->is_home() && !($query->is_paged()) && $query->is_main_query() ) {
    $query->set( 'cat', '2');
    }
    else{
        //Manually determine page query offset (offset + current page (minus one) x posts per page)
        $page_offset = $offset + ( ($query->query_vars['paged']-1) * $ppp );
        //Apply adjust page offset
        $query->set('offset', $page_offset );
    }
}
add_action ('pre_get_posts', 'my_home_category');