htaccess ErrorDocument with Subdomains

So I recently tried to make my own error page (404). It looks good, but after hours of searching how to add it (didn't work), I give up... How can I get the same ErrorDocument on every subdomain with this structure?:

/var/www
  -main site
    -.htaccess
  -subdomain
    -.htaccess
  -error
    -error***.html (*** = which error, 404 etc)
    ...

I want every (sub)domain to have the same ErrorDocument. I tried inheriting (.htaccess in /var/www), but it gives me a error (additionally an error encountered trying to use a ErrorDocument or something like that).

How can i do this? Every try either just gave me "/error/404.html" as html or encountered an error. The document root is in the Subdomains folder, that's the problem. And going up a folder in htaccess doesn't work (../error).

Please help 😓