Installation points to wrong URL

While doing the installation of my new blog I noticed something strange.

My blog is hostet on a webhoster which doesn’t provide a top-level domain. therefore I point one of my top-level domain to the worpress folder. But this doesn’t work.

for example:
Top-Level domain: https://toplevel.com/
installation: https://user.hoster.com/wordpress/

so when I access my top-level domain I get the “you got no config file” message which is expected on a fresh installation. But when I click on the “Create a Configuration File” button I get redirected to my installation path which shouldn’t be because I want to access the blog with my top-level domain. So here is the first problem. It seems I can’t access my installation with the top-level domain.

So I installed it “normally” and changed the URL in the settings to my top-level domain. But then I get on some buttons (plugins, save changes in settings and others) redirected to the following URL: https://toplevel.com/wordpress/ which isn’t right at all.

I tried some hints on a solution with resetting the permalinks (which redirected me to the wrong address), editing the .htaccess file which is correct.

So since I wrote here I’m out of options and it’s quite annoying. Any Ideas?