Multisite domain mapping - can't access dashboard - redirected to login screen #410
Description
Hi there, we've migrated our website to Siteground using our domain name https://asimplyfablife.com/ and have our blog as a subdirectory under https://asimplyfablife.com/asfl2/. We'd like to keep the UI as our old domain https://asimplyfab.life. We've changed the DNS and have added the domain to Siteground and allocated an SSL certificate.
When I add this domain under Sites, Settings, Site Address (URL), purge the cache and try to access the dashboard, it redirects me to a login page (https://asimplyfab.life/wp-login.php?redirect_to=https%3A%2F%2Fasimplyfab.life%2Fwp-admin%2F&reauth=1) and doesn't acknowledge my login credentials. I was able to reset my password, and when I log in, I get directed to the primary site https://asimplyfablife.com/. When I try to access the dashboard again for our blog at /asfl2/ the loop repeats itself.
I contacted Siteground support and they suggested using a plugin like Mercator. Which I installed following their github instructions. After adding a 'fix' to get the alias tab showing, when I clicked on it - it opened a blank page. I manually deactivated all plugins and themes one-by-one to see if there was a conflict - no change.
I thought it might be because this subsite uses the Twenty Twenty Five theme, where the others are using Elementor - but the domain mapping also does the loop with the other subsites.
Thinking it might be a cookie error, I also added define( 'COOKIE_DOMAIN', $_SERVER['HTTP_HOST'] ); to the wp-config.php but it hasn't made a difference either.
I'd love to log in and access our blog via the primary site, and for visitors to see our old urls.
How can I resolve this please? Many thanks, Victoria