mask browser url when coming from different domain

I have installed wordpress successfull on a server which is called
domain-1.com
Now I make a redirect from another domain from a different server called
domain-2.com. Works fine so far.

But I want the browser to show always just domain-2.com, but it always shows the domain-1.com/subdir/subdir2

I tried to modify htacces file, tried the Site url in wordpress config, tried to put it in an iframe, tried to manipulate the url via javascript. Nothing works.
It always looks like this:
domain-1.com/subdir/subdir2

When I change the Site-Url in WordPress config, the site doesn’t work anymore because I can not upload the necessary files to the domain-2.com server.
Wordpress must stay completely on domain-1.com server.

When I use the javascript trick

<script>
parent.history.replaceState("object", "www.domain-2.com", "www.domain-2.com");
</script>

I can make the browser url look like:
domain-1.com/subdir/subdir2/www.domain-2.com
But I have no idea why I can replace the first part of the url.
So also no way.

Any ideas how I can solve my problem ?