Install microg, add your Google account,install google drive and then reinstall Whatsapp. It should find the backup now. But when restoring, there seems to be a timeout or similar. At least in latest version. And at least in my case 
Install microg, add your Google account,install google drive and then reinstall Whatsapp. It should find the backup now. But when restoring, there seems to be a timeout or similar. At least in latest version. And at least in my case