POLY API: google sign-in doesnt work on Android

i am making an Augmented Reality App, where i need to load different Assets at Runtime. With the Poly Api I achieved to load public assets into my AR-App at Runtime.  The thing is, i want to load private Assets from Poly into my App, therefor i need to sign-in in my App with my google account. But that's the problem, it doesnt even open the browser window.

I tested it in the Unity game window, there i can sign in and load my private assets as well. Poly says, that " To access private assets, you need to get an OAuth client ID."  i have a client id, otherwise it wouldn't work in the unity game window. Has anybody an idea, how to use it on Android?

Thanks

Julia