I'm not the best at these things but it seems that 10.2.1 works and my old i4 also works so I don't see why the 5s shouldn't. They are still a current phone and are still being supported. Are you sure you are not missing something? Can you post a screen shot or something that you are getting? do other functions work?
Don't know how to post a screen shot from my iPhone here. The GS-911 app does state that it IS connected to the WiFi network, and when I go to "Series" to select the R1200GS Adventure...that seems to work just fine. I assume that when you select the model, the GS-911 WiFi is communicating with the bike (maybe not?)
Anyway, after that, NO other functions return anything. When I query ANY function, I get the following message:
Error: Couldn't get data for function (,Code:0)
That has happened with my iPhone, iPad (also iOS 10.2.1), and MacBook Pro (using Safari browser). My wife's iPad (running iOS 9.3.5) works just fine, as does my PC laptop via USB cable.
And everything used to work just fine with all those mobile devices. I'm trying not to be negative, but the last message from HEX was "we'll let you know when we have an update for the App." Doesn't sound like a quick fix.
I do appreciate all the input from you folks.