custom post types don’t appear in RSS

They don’t normally show there
That is how they are supposed to work.
Each CPT has a feed of it’s own by default
Everything in WP has a feed it seems!

But if you want them in your main feed

this can go in your functions.php

// ADDS POST TYPES TO RSS FEED
function myfeed_request($qv) {
    if (isset($qv['feed']) && !isset($qv['post_type']))
        $qv['post_type'] = array('ve_products', 'post');
    return $qv;
}
add_filter('request', 'myfeed_request');

You see this line:

$qv['post_type'] = array('ve_products', 'post');

That includes normal posts and my CPT of ve_products in my main feed

You would swap out that for your CPT, if you have more CPTs, add them into that array