Wordpress blogpost subdirectory - wordpress

Question

I have a blog subdirectory
public_html/blog
the site can be accessed like this
https://example.com/blog
by default, the blogposts are accessed like this
https://example.com/blog/index.php/hello-world
I want to remove the index.php and access the posts like this
https://example.com/blog/hello-world
I have this .htaccess file from where i looked into how to install a wordpress subdirectory
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /blog/
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /blog/index.php [L]
</IfModule>
https://www.wpbeginner.com/wp-tutorials/how-to-install-wordpress-in-a-subdirectory-step-by-step/
However when I tried to change it via
Settings -> Permalinks -> Common Settings -> Post name
I end up with a 404 from the main wordpress installation. I have a faint idea that this might be an issue with the htaccess? But i don't really get how to fix it.

Answer 1

when I remove the index.php with the settings it gets redirected to the 404 of the main wordpress installation
That's because the rewriting for that main WordPress installation rewrites everything that does not match a physically existing file or folder, to its own index.php. (Which then goes and looks up the requested URL in its database, doesn't find any matching content, and responds with a 404.)
/blog/index.php/hello-world matches the existing index.php in that folder (the rest is considered PATH_INFO), so the rewriting of the main WP leaves it alone.
But /blog/hello-world does not match anything that exists in the file system.
So you need to exclude any paths starting with /blog/ from the rewriting inside your main WP installation.
https://stackoverflow.com/a/15297697/1427878 shows the basic way to do that, although there the content between the # BEGIN WordPress/ # END WordPress comments gets modified (which one should not do, because that part will be overwritten when the permalink settings get flushed in the backend.)
Better to do it outside of those. Try and put this at the very top of your .htaccess of the main WP installation:
RewriteEngine On
RewriteRule ^blog/ - [L]
For any path that starts with /blog/ (leading slash has already been removed from the relative path that RewriteRule matches against), simply do no rewriting at all, and make this the last rule to be processed at the same time.