permalinks – WordPress in submap of non-wordpress site

My advice is asked to have permalinks active for WordPress, where WordPress is installed in a sub directory of a website, which is not part of WordPress. (Only the blog part/url is WordPress)

If we simply enable the permalinks, WordPress reports no errors or warnings, and generates the correct URL’s. However, the pages can not be reached anymore (404). Probably because WordPress updates it’s own .htaccess in it’s own blog directory and the website deals with the .htaccess in the root directory.

So this is essential different from the default “WordPress in its own directory” procedure, because we can not set the homepage to the domain root. (WordPress must concider the blog directory/url as its main directory)
I hope I made the situation clear.

We already tried different things. We copied the content of the wordpress .htaccess to the .htaccess in the root directory. Both, with RewriteBase/ and RewriteBase/blog/ but no success.
And since I’m not much of an htaccess expert I don’t know what to try next.

Anyone?

The website btw is:
https://www.3develop.nl/
WordPress resides in https://www.3develop.nl/blog