I'm unable to reproduce an issue which only seems to occur on App Review devices. It's mainly stuck at a loading screen when trying to sign in with GameCenter.
I'm using Microsoft Azure PlayFab as a game backend. I've tested my game on two iPads and three different iPhone models. They're all able to connect to the server. No debug build was installed on those devices via Xcode. I used the same build which was rejected, freshly installed via TestFlight on the internal test track.
Looking at my server logs, I can't even find one single request sent from the App Review device. This can only mean that there is some connectivity issue between their device and the server (e.g. a firewall or proxy).
I've also implemented a retry mechanism so in case the request times out, it would try it again and after three retries it would eventually display an error message and further details.
How shall I figure this issue out, if the only thing the reviewers provided me is a screenshot of the loading screen?