How to Test an app in Alpha (Android) on a tablet

I have:

• ... already tested an App in Alpha earlier. It seems there where conceptional changes meanwhile. (All gets better :-))
• A new app version published for Alpha-Testing
• I have a User in a testing group assigned and Testing Group is assigned to Alpha-Test
• a link for testing the app and sent the link to the testing User.
• User klicks on the link and brwoser opens. User signs-in with User-Account which is Member of the testing Group and gets the error-message: "To access this content you need to install and set up a web browser app."

So now I am unable or too stupid to simply test an app.

Can someone help me ?