Can't connect Google Play Games Services - No Solution

My app is in internal test. Testers can download and play the game but the app does not connect with Google Play Games. Everything is in order. When you click Google Play Games, you can see the app, the time when it’s last played etc. Oauth ID’s are correct as well.

I looked almost every topic about this on answer hub and forums but couldn’t find a legit solution. Do I really have nothing to do with this?

My app was linked to Firebase and I connected signing keys to Firebase as well. It didn’t work. Then I publish my Game Services and it now connects to Google. I guess, since it’s shipping build, Game Services needs to be published.

My game is already published and in Google console allow saved games are on but still I’m facing the same issue as mentioned above

Google + will be shutting down and that will effect a lot of developers who are using these API for signing in purposes. How ever everything is all quite on the western front until they all start freaking out about it. There is like 1 post about these but on the out side the whole world knows that Google + is going to shut down. lol. I post a question about what can be done to prevent these since I got Google warning E-mail and question hasn’t been answered yet. It’s like looking for Walmart in the desert.