BuddyPress.org

Three ways to go about it depending on preference and how site is used:

1. define ( \'BP_ENABLE_MULTIBLOG\', true );

add this to your wp-config.php after the DB collate info will allow BP to function on different blogs of your install

Reference: https://codex.buddypress.org/how-to-guides/changing-internal-configuration-settings/ :

OR

2. You can copy source view code of the navigation in home site and paste it to replace relative navigation links in header.php your child theme to get the URL of home page navigation correctly. Caveat – you would have to do this every time you add new navigation link in home page.

OR

3. You could add the following codes before and after the navigation menu in header.php of your child theme

before main nav assuming BP site is at main blog

<?php switch_to_blog(\'1\') ?>

then after main nav

<?php restore_current_blog(); ?>