Deeplinks can work without the need to login, see the documentation on ‘Do not force login action’ here: https://docs.mendix.com/appstore/modules/deep-link/#34-adding-the-configuration-overview-snippet-the-custom-admin-page
All you would need to do is take the tentent info from the url as the input parameter for the deeplink microflow and make sure that based on this data the user is redirected to the correct page. Or even better maybe directed to the same page but with elements like logo’s etc that are set dynamically so that yoou do not need a different page for every tenant.
I do believe that using deeplink is the way forward and you’ll need to see what the issue is in your configuration.
The deeplink should work for customized login pages, like Erwin said, but you’re getting in trouble when users logout or when browsers are redirected to the (?) login page when sessions are terminated or the application is restarted.
You may want to look into setting up multiple login.html files in the /theme/web folder of your project. Users then can login at <your_app_url>.login-tenant1.html, <your_app_url>.login-tenant2.html etc. Using the originURI cookie users should then be redirected to their own login pages when their session ends. I think Mendix sets this cookie automatically in login.js, otherwise you’d need to do that yourself using javascript in your login.html files.