Login loop (site URL). No loop (home URL). Customizer broken

Hi,

I have my wordpress installed (godaddy) at https://www.mysite.com/subdir

I have a domain, https://www.mydomain.com at another registrar for which I have set up the correct name servers, DNS etc. to be configured with https://www.mysite.com/subdir

Everything seemed to be working fine. I could browse to https://www.mydomain.com and it was working, however https://www.mydomain.com/contact was not working correctly (400 internal error).

Therefore, I had changed my permalinks settings (.htaccess file) as suggested by WordPress and changed my site URL in the General settings. The link https://www.mydomain.com/contact was working, all seemed fine now.

However, I tried to login to https://www.mydomain.com/wp-admin and I got stuck in a loop which kept on bringing me to the login page.

I have found many topics on this forum and applied all solutions I could find, however the problem still persists.

Now I am at at the following point.

https://www.mysite.com/subdir/wp-admin logs me into my dashboard
https://www.mydomain.com/wp-admin gets me stuck into the loop
After login successful login with https://www.mysite.com/subdir/wp-admin: Dashboard/Customizer (I use the Parallax Theme) gives me the login page and gets me stuck in the loop. This causes me being unable to apply any changes through the Customizer.

I have added the following in my wp-config file (suggestions of one of the topics here). I have also tried everything without adding the following information to the wp-config file.

define(‘WP_HOME’,’https://mydomain.com’);
define(‘WP_SITEURL’,’https://mysite.com/subdir’);
note: I initially thought WP_HOME must have the directory were WP is installed and WP_SITEURL must have the domain, but then I browsed to the General settings in my dashboard and it appeared to be the other way around (??). I tried both ways, but the other way around it got me stuck in a loop for both login options.
define(‘DOMAIN_CURRENT_SITE’, ‘mydomain.com’ );
define( ‘COOKIE_DOMAIN’, ‘mydomain.com’ );
define(‘ADMIN_COOKIE_PATH’, ‘/’);

My .htaccess file I have adjusted as per the suggestions of Settings/Permalinks where I have changed RewriteBase /subdir to RewriteBase / and RewriteRule . subdir/index.php [L] to RewriteRule . /index.php [L]

I am baffled cause I can login to my dashboard with the link https://www.mysite.com/subdir/wp-admin, but NOT with https://www.mydomain.com/wp-admin (stuck in the loop)

When I browse to https://www.mysite.com/subdir/wp-admin I get the following URL in my browser
https://mysite.com/subdir/wp-login.php?redirect_to=http%3A%2F%2Fmysite.com%2Fsubdir%2Fwp-admin%2F&reauth=1

When I browse to https://www.mydomain.com/wp-admin I get the following URL in my browser
https://mysite.com/subdir/wp-login.php?redirect_to=http%3A%2F%2Fwww.mydomain.com%2Fwp-admin%2F&reauth=1

I have also tried renaming/deleting my .htaccess file and changing the permissions but this has not worked either.

Does anyone have any idea where the problem could be? I don’t mind if only https://www.mysite.com/subdir/wp-admin allows me to login to my dashboard, given the fact that everything else is working and I can use the Customizer to adjust the website. Unfortunately this is not the case