Google Play Game Multiplayer Notifications Failure

I am trying to create a cross platform turn-based game which works on both iOS and Android through Google Play, and to do this I am currently using the iOS/Android Play Game Services plugin by Prime31. The problem is that notifications are not working. Any advice on getting Google Play iOS notifications working for turn-based games would be greatly appreciated, whether general or specific.

The tricky thing about this problem is that it's so hard to troubleshoot or get any information about, which makes it less than ideal for asking help with. Most of the problem happens either in the guts of the plugin (which don't appear in the logs) or else on Google's servers which are completely opaque and inaccessible as far as I can tell. It's probable that I've been doing something wrong, but I've tried making a thousand variations of the code, resubmitting the certificates many times in many ways on both consoles, reinstalling the plugin, upgrading unity, and the result is always precisely the same: no notifications from Google. Support is tricky because Prime31 defers responsibility to Google, and Google doesn't support anything involved with iOS. I’ve tried sending the notification manually from my computer with the same certificate I sent Google, and it went through just fine, so the apple stuff is configured correctly. The problem appears to be between the app and Google in the device token registration phase, or else between Google and APNS.

If anyone has any experience with anything to do with this, specifically getting Google Play Game Services iOS notifications to work with either Prime31’s plugins or by any other means, or even if you have general advice for solving thorny issues such as this and getting support, please let me know.

Thanks.

OSX 10.11.2 Unity 5.3.5 Prime31 Google Play Game Services iOS plugin 2.1.4 XCode 7.3.1 iOS 9.1